diff --git a/fastNLP/core/dataloaders/jittor_dataloader/fdl.py b/fastNLP/core/dataloaders/jittor_dataloader/fdl.py
index da896cf8..349fb444 100644
--- a/fastNLP/core/dataloaders/jittor_dataloader/fdl.py
+++ b/fastNLP/core/dataloaders/jittor_dataloader/fdl.py
@@ -24,7 +24,7 @@ from fastNLP.core.dataset import DataSet as FDataSet
 class _JittorDataset(Dataset):
     """
     对用户传的dataset进行封装，以便JittorDataLoader能够支持使用自定义的dataset
-    
+
     """
 
     def __init__(self, dataset) -> None:
@@ -37,7 +37,7 @@ class _JittorDataset(Dataset):
             item = item.tolist()
         return (item, self.dataset[item])
 
-
+          
 class JittorDataLoader:
     """
     提供给 ``jittor`` 框架使用的 ``DataLoader`` 函数，``JittorDataLoader`` 提供了 ``Collator`` 来自动检测 dataset 的每个 field 是否可 pad，
diff --git a/fastNLP/core/dataloaders/mix_dataloader.py b/fastNLP/core/dataloaders/mix_dataloader.py
index d6f6a9be..29b0cd0b 100644
--- a/fastNLP/core/dataloaders/mix_dataloader.py
+++ b/fastNLP/core/dataloaders/mix_dataloader.py
@@ -2,13 +2,14 @@ __all__ = [
     'MixDataLoader'
 ]
 
-from typing import Optional, Callable, List, Union, Tuple, Dict, Sequence
+from typing import Optional, Callable, List, Union, Tuple, Dict, Sequence, Mapping
 
 import numpy as np
 
 from fastNLP.core.dataset import DataSet, Instance
 from fastNLP.core.samplers import PollingSampler, MixSequentialSampler, DopedSampler
 from fastNLP.envs.imports import _NEED_IMPORT_TORCH
+from fastNLP.core.collators import Collator
 
 if _NEED_IMPORT_TORCH:
     from torch.utils.data import DataLoader, Sampler
@@ -18,12 +19,13 @@ else:
 
 class _MixDataset:
     """
-    将所有数据集当成一个混合大数据集来对待，实现的__getitem__能区别每个数据idx
+    将所有数据集当成一个混合大数据集来对待， 在 __getitem__() 能根据输入的 idx 来判断属于哪个小数据并返回其 ds_index
 
     """
+
     def __init__(self, datasets: list = None) -> None:
         """
-        :param datasets: 数据集的列表
+        :param datasets: 实现了 __getitem__() 和 __len__() 的对象的序列
         """
         self.datasets = datasets
         # 记录每个数据集的长度索引， 以便根据idx定位数据集的位置
@@ -35,7 +37,7 @@ class _MixDataset:
 
     def __getitem__(self, idx: Union[int, List[int]]) -> Union[Tuple[Instance, int], Tuple[DataSet, int]]:
         """
-        根据index索引获取数据
+        根据index索引获取数据， 能够跟 idx 的范围定位属于哪个小数据并返回
 
         :param idx: 整数类型的index或者列表
         :return:
@@ -69,8 +71,9 @@ class _MixCollateFn:
     存在多个auto_collate和多个collate_fn时候，对一个批次数据集应用哪个auto_collate和collate_fn的问题
 
     """
-    def __init__(self, collate_fns: Optional[Union[List[Callable], Callable]] = None,
-                 auto_collators: Optional[List[Callable]] = None) -> None:
+
+    def __init__(self, collate_fns: Union[List[Callable], Callable]) -> None:
+
         if isinstance(collate_fns, Sequence):
             self.collate_fns = lambda idx, lst: collate_fns[idx](lst)
         elif callable(collate_fns):
@@ -78,96 +81,124 @@ class _MixCollateFn:
         else:
             self.collate_fns = lambda idx, lst: lst
 
-        self.collate_fns = collate_fns
-        self.auto_collators = auto_collators
-
     def __call__(self, ins_list: List) -> Dict:
         """
         调用一次该方法，我们将ins_list视为同一个数据集采样出来的，故ds_index只能为一种
+
         :param ins_list:
         :return:
         """
         _ins_list, _ds_index = [], 0
         for ins, _ds_index in ins_list:
             _ins_list.append(ins)
-        # auto_collate先处理
-        if self.auto_collators is not None:
-            _ins_list = self.auto_collators[_ds_index](_ins_list)
         _ins_list = self.collate_fns(_ds_index, _ins_list)
         return _ins_list
 
 
 class MixDataLoader(DataLoader):
     """
-    针对一下三种情况提供的MixDataLoader:
-        1. 给定datasets集合或者列表，顺序采样datasets，处理采样完首个dataset后取出第二个dataset，重复上面过程直至datasets取完。
-        2. 给定datasets集合或者列表，随机采样这个datasets的任意一个数据集组合成一个混合的batch返回给用户，直至datasets所有数据集采样完。
-        3. 给定datasets集合或者列表，轮流采样datasets：即是循环遍历datasets，每取出一个dataset采样一个batch的数据，然后取出下一个dataset
-           采样一个batch数据，重复上述过程直至某个dataset采样结束或者所有dataset采样结束。
+    针对一下四种情况提供的 ``MixDataLoader``， 目前只支持 ``torch`` 框架的版本， 其中 mode 的取值范围为 ``['sequential', 'mix', 'polling', "Sampler"]``:
+
+        * 当 mode 为 ``'sequential'`` 时，``MixDataLoader``  将 datasets 的序列或者字典视为一个混合大数据集， 按照 datasets 数据集序列或者字典的顺序一个
+        接一个的 sample 完所有数据。
+        * 当 mode 为 ``'mix'`` 时， ``MixDataLoader``  将 datasets 的序列或者字典视为一个混合大数据集， 然后根据用户输入的 idx 序列随机sample
+        混合数据集 datasets 的数据组成一个 batch 序列返回。
+        * 当 mode 为 ``'polling'`` 时， ``MixDataLoader`` 按照 datasets 数据集的顺序， 先从第一个数据集采样一个 batch 的数据返回，
+        再从第二数据集采样一个 batch 数据返回， 直至最后一个数据集采样一个 batch 数据返回后再从第一个数据采样第二个 batch 数据返回，直至所有的数据集都被轮询的采样完。
+        * 当 mode 为 ``"Sampler"`` 时， 该 Sampler 是实现 __iter__() 的实例化对象， 其功能是每次 iter 时返回一个 batch 序列， 其类型为 List[int];
+        且 Sampler 必须将输入的 datasets 视为一个混合大数据集， 其 index 范围为 ``0<idx<len(datasets[0])+...+len(datasets[x])``, 然后参数
+        sampler, drop_last, ds_ratio 均无效。
+
     """
-    def __init__(self, datasets: Union[List, Dict] = None, mode: Union[str, "Sampler"] = 'sequential',
-                 collate_fn: Union[List[Callable], Callable, Dict[str, Callable]] = None,
-                 sampler: Union[List["Sampler"], Dict[str, "Sampler"]] = None,
+
+    def __init__(self, datasets: Dict = None, mode: Union[str, "Sampler"] = 'sequential',
+                 collate_fn: Union[str, Callable, Dict[str, Callable]] = 'auto',
+                 sampler: Union[Dict[str, "Sampler"], str, None] = None,
                  num_workers: int = 0, batch_size: int = 16, drop_last=False,
-                 ds_ratio: Union[str, List[float], None, Dict[str, float]] = None,
-                 pin_memory: bool = True) -> None:
+                 ds_ratio: Union[None, str, Dict[str, float]] = None,
+                 pin_memory: bool = False) -> None:
         """
 
-        :param datasets: dataset的列表
-        :param mode: mode包括四种类型，前三种分别为"sequential", "mix", "polling"分别代表上述三种情况，
-                    当mode为Sampler时为用户定制，此时sampler，ds_ratio，batch_size，drop_last失效，此时Sampler应该是一个可迭代
-                    对象，每次迭代返回的是List[int]
-        :param collate_fn: 对取得到的数据进行打包的callable函数，
-                         当其为callable类型时候，所有数据集采样的数据都会经过这个函数；
-                         当其为List[Callable]类型时，datasets也应该为List；会根据每个数据集__getitem__返回的idx判断当前数据对应的Callable函数，
-                         其对应关系与datasets位置匹配；
-                         当其为Dict[str, Callable]类型时， datasets也是Dict类型且一一对应。
-        :param sampler: sampler是datasets每个数据集内部采样的实例化sampler对象
-                        sampler为None时候，datasets包含的每个dataset都会初始化一个sequentialSampler用于采样;
-                        sampler为List[Sampler]，则datasets也为List，且一一对应
-                        sampler为Dict[str, Sampler], datasets也是Dict类型且一一对应。
-        :param num_workers: 进程的数量，当num_workers=0时不开启多进程
-        :param batch_size: 批次大小, datasets的所有数据集batch_size一致
-        :param drop_last: 是否去掉最后一个不符合batch_size的数据
-        :param ds_ratio: 当ds_ratio为None，原有数据集不进行扩充
-                        当ds_ratio为'truncate_to_least'时，以datasets的最短数据集为基准，将其他数据集截断到一样长度
-                        当ds_ratio为'pad_to_most'时，以datasets的最长数据集为基准，将最短数据集重采样到最长数据集长度一致为止
-                        当ds_ratio为List[float]时，datasets也为List，ds_ratio的每一个参数都是datasets每个数据集应该采样的倍数，
-                        其大于0，可以超过1，将数据集重采样翻倍即可
-                        当ds_ratio为Dict[str, float]时，datasets也为Dict，参数相互对应。
+        :param datasets: 实现了 __getitem__() 和 __len__() 对象的序列或者字典。
+        :param mode: mode 控制 ``MixDataLoader`` 运行模式。 mode 的取值范围为 ``['sequential', 'mix', 'polling', "Sampler"]``:
+
+            * 当 mode 为 ``'sequential'`` 时，``MixDataLoader``  将 datasets 的序列或者字典视为一个混合大数据集， 按照 datasets 数据集序列或者字典的顺序一个
+            接一个的 sample 完所有数据。
+            * 当 mode 为 ``'mix'`` 时， ``MixDataLoader``  将 datasets 的序列或者字典视为一个混合大数据集， 然后根据用户输入的 idx 序列随机sample
+            混合数据集 datasets 的数据组成一个 batch 序列返回。
+            * 当 mode 为 ``'polling'`` 时， ``MixDataLoader`` 按照 datasets 数据集的顺序， 先从第一个数据集采样一个 batch 的数据返回，
+            再从第二数据集采样一个 batch 数据返回， 直至最后一个数据集采样一个 batch 数据返回后再从第一个数据采样第二个 batch 数据返回，直至所有的数据集都被轮询的采样完。
+            * 当 mode 为 ``"Sampler"`` 时， 该 Sampler 是实现 __iter__() 的实例化对象， 其功能是每次 iter 时返回一个 batch 序列， 其类型为 List[int];
+            且 Sampler 必须将输入的 datasets 视为一个混合大数据集， 其 index 范围为 ``0<idx<len(datasets[0])+...+len(datasets[x])``, 然后参数
+            sampler, drop_last, ds_ratio 均无效。
+
+        :param collate_fn: 用于从 dataset 取到的一个 batch 数据进行打包处理的 Callable 函数。 其取值可以为 ``['auto', Callable, List[Callable], Dict[str, Callable]]``:
+
+            * collate_fn 为 ``'auto'`` 时, ``MixDataLoader``  datasets 序列或者dict 初始化一个 :class: `~fastNLP.core.collators.Collator`  作为其默认值，
+            需要注意的是只有当 datasets 包含的所以 dataset 的数据都为 ``List`` 或者 ``Dict`` 类型时才能使用。否则只能用户自己定义 collate_fn .
+            * collate_fn 为  ``Callable`` 时， 该 collate_fn 会被 datasets 序列或者dict 的所有数据所共享。该 Callable 函数应当接受一个 batch 参数作为输入，
+            batch 是一个 List 对象且 List 中的每一条数据都是 dataset 的一条数据；该 Callable 函数还应当返回一个对象。
+            * collate_fn 为 ``Dict[str, Callable]`` 时， datasets 的 key 必须和 callable_fn 的 key 一致。 ``MixDataLoader`` 会将 ``collate_fn[key]``
+            用到 ``datasets[key]`` 的数据集上。 ``collate_fn[key]`` 是一个 Callable 对象。
+
+
+        :param sampler: 实现了 __len__() 和 __iter__() 的实例化对象，其 __iter__() 方法每次都会返回 dataset 的一个下标 index ，其取值范围为
+        ``[None, str, Dict[str, "Sampler"]]``:
+
+            * sampler 为 ``None`` 时， ``MixDataLoader`` 默认初始化 ``torch`` 的 ``SequentialSampler`` 作为默认值。其功能时顺序返回 dataset 的下标。
+            * sampler 为 ``str`` 时， sampler 选择范围为 ``[rand, seq]``。当 sampler 为 ``rand`` 时，``MixDataLoader`` 默认初始化 ``torch`` 的  ``RandomSampler``
+            作为默认值， 其功能时随机采样 dataset 的下标并返回。 当 sampler 为 ``seq`` 时， ``MixDataLoader`` 默认初始化 ``torch`` 的 ``SequentialSampler`` 作为默认值。其功能时顺序返回 dataset 的下标。
+            * sampler 为 ``Dict[str, "Sampler"]`` 时， ``Sampler`` 为用户定义的实现了 __len__() 和 __iter__() 的实例化对象。 其每次 iter 必须返回一个 int 下标。
+            Dict 的 str 必须和 datasets 的 key 一致。 也即是 ``Dict[str, Sampler] `` 为 datasets 字典的每个 dataset 初始化勒一个 Sampler。
+
+        :param num_workers: 当 ``num_workers > 0`` 时, ``MixDataLoader`` 会开启 num_workers 个子进程来处理数据， 可以加快数据处理速度，但同时
+        也消耗大量内存。 当 ``num_workers=0`` 时， 不开启子进程。 默认为 ``0``。
+        :param batch_size: 批次大小，默认为 ``16`` 且当 batch_sampler 为 None 有效。 且 datasets 上所有 dataset 的 batch_size 一致。
+        :param drop_last: 当 ``drop_last=True`` 时，``MixDataLoader`` 会扔掉 datasets 中 每个 dataset 最后一个长度小于 ``batch_size`` 的 batch 数据;
+        若 ``drop_last=False`` , 则会返回该 batch 数据。 默认为 ``False`` 。
+        :param ds_ratio: ``ds_ratio`` 是控制 datasets 怎么组成一个混合大数据集的重要参数， 其取值为 ``[None, 'truncate_to_least', 'pad_to_most', List[float], Dict[str, float]]``:
+
+            * ds_ratio 为 ``None``, datasets 数据集序列或字典不进行数据扩充处理。
+            * ds_ratio 为 ``'truncate_to_least'``, datasets 数据集序列或字典会计算得到 datasets序列中 dataset 最断长度 ``mix_len``， 其他数据集会被切断
+            到最短长度``mix_len``。这种切断不是物理上切断，``MixDataLoader`` 会根据 sampler 不同来采样数据集到指定的最短长度``mix_len``。
+            * ds_ratio 为 ``'pad_to_most'``, datasets 数据集序列或字典会计算得到 datasets序列中 dataset 最大长度 ``max_len``, 其他其他数据集会扩充
+            到最大长度``mix_len``。这种扩充不是物理上扩充， ``MixDataLoader`` 会根据 sampler 不同来重采样 dataset 到指定的最大长度``max_len``。
+            * ds_ratio 为 ``Dict[str, float]`` 时， datasets 类型也必须为 ``Dict[str, DataSet]``, 其 key 一一对应。 ds_ratio 的 value 是任意大于 0 的浮点数，
+            代表着 datasets 的 value 数据进行扩充或者缩减的倍数。
         """
-        # 如果dataset为Dict，则其他参数如collate_fn必须为Dict或者Callable,
-        if not isinstance(datasets, Dict) and (isinstance(collate_fn, Callable) or isinstance(collate_fn, Dict)) and \
-                isinstance(sampler, Dict):
-            raise ValueError(f"")
-
-        if isinstance(collate_fn, list):
-            if len(collate_fn) != len(datasets):
-                raise ValueError("the length of collate_fn != datasets!!")
-
-        if isinstance(sampler, list):
-            if len(sampler) != len(datasets):
-                raise ValueError("the length of sampler != datasets!!")
-
-        # Dict类型转化为List，以便于_MixCollateFn处理
+        # sampler 为 dict，则判断是否与 datasets 的 key 相同
+        if isinstance(sampler, Dict):
+            for key in datasets.keys():
+                if not sampler[key]:
+                    raise ValueError(f"the key:{key} of datasets is not in sampler, where sampler is a dict!")
+        # collate_fn 为 dict，则判断是否与 datasets 的 key 相同
+        if isinstance(collate_fn, Dict):
+            if mode == 'mix':
+                raise ValueError(f"mode: {mode} do not support collate_fn is Dict, please use callate_fn=Callable or 'auto'")
+            for key in datasets.keys():
+                if not collate_fn[key]:
+                    raise ValueError(f"the key:{key} of datasets is not in collate_fn, where collate_fn is a dict!")
+
+        if isinstance(collate_fn, str) and collate_fn == 'auto':
+            date_type = None
+            for idx, ds in enumerate(datasets.values()):
+                if idx == 0:
+                    date_type = type(ds[0])
+                if type(ds[0]) != date_type or not (isinstance(ds[0], List) or isinstance(ds[0], Mapping)):
+                    raise ValueError(f"when you use callate_fn={collate_fn}, all dataset must be list or dict。"
+                                     f"But dataset {idx - 1} data type is {date_type}, dataset {idx} data type is {type(ds[0])}")
+
+            collate_fn = Collator(backend='torch')
+
+        # Dict 类型的 collate_fn 转化为 List，以便于 _MixCollateFn 里面根据 idx 定位 dataset
         if isinstance(collate_fn, Dict):
             collate_fn = [fn for _, fn in collate_fn.items()]
 
-        # 由于datasets可能是FastNLP类型的dataset或者是交杂的, 故需要检测
-        if isinstance(datasets, Dict):
-            dataset = [ds for _, ds in datasets.items()]
-        else:
-            dataset = datasets
-        auto_collators = []
-        for per_ds in dataset:
-            if isinstance(per_ds, DataSet):
-                auto_collators.append(per_ds.get_collator())
-            else:
-                # 如果没有对应的collator就设置一个不做任何操作的collator
-                auto_collators.append(lambda x: x)
-
-        # List类型的collate_fn只有两种情况，需要对其进行包裹
-        collate_fn = _MixCollateFn(collate_fn, auto_collators)
+        dataset = [ds for _, ds in datasets.items()]
+
+        # 对 collate_fn 进行包裹， 统一处理 collate_fn 不同情况下使用的问题
+        collate_fn = _MixCollateFn(collate_fn)
+
         if mode == 'sequential':
             batch_sampler = MixSequentialSampler(datasets, batch_size=batch_size, sampler=sampler,
                                                  drop_last=drop_last, ds_ratio=ds_ratio)
diff --git a/fastNLP/core/samplers/mix_sampler.py b/fastNLP/core/samplers/mix_sampler.py
index 0aa543be..774b5c7c 100644
--- a/fastNLP/core/samplers/mix_sampler.py
+++ b/fastNLP/core/samplers/mix_sampler.py
@@ -21,9 +21,9 @@ class MixSampler:
     mix_sampler的基类
     """
 
-    def __init__(self, dataset: Union[List, Dict], batch_size: int = None,
-                 sampler: Union[List["Sampler"], Dict[str, "Sampler"], None, str] = None,
-                 ds_ratio: Union[str, List[float], Dict[str, float]] = None,
+    def __init__(self, dataset: Dict, batch_size: int = None,
+                 sampler: Union[Dict[str, "Sampler"], None, str] = None,
+                 ds_ratio: Union[str, Dict[str, float]] = None,
                  drop_last: bool = False, rank: int = -1, word_size: int = -1) -> None:
         """
 
@@ -32,9 +32,12 @@ class MixSampler:
         :param sampler: 实例化好的sampler，每个dataset对应一个sampler对象
         :param drop_last: 是否去掉最后一个batch的数据，其长度小于batch_size
         """
-        # 如果dataset为Dict，则其他参数如collate_fn必须为Dict或者Callable,
-        if isinstance(dataset, Dict) and isinstance(sampler, List):
-            raise ValueError(f"{sampler} must be dict")
+        # sampler 为 dict，则判断是否与 datasets 的 key 相同
+        if isinstance(sampler, Dict):
+            for key in dataset.keys():
+                if not sampler[key]:
+                    raise ValueError(f"the key:{key} of datasets is not in sampler, where sampler is a dict!")
+
         if batch_size <= 0:
             raise ValueError("batch_size should be a positive integer value, "
                              "but got batch_size={}".format(batch_size))
@@ -46,15 +49,7 @@ class MixSampler:
             raise ValueError("if rank>=0 and word_size>=0, sampler must be str")
 
         if sampler is None and (word_size < 0 or rank < 0):
-            if isinstance(dataset, List):
-                self.sampler = [SequentialSampler(ds) for ds in dataset]
-            elif isinstance(dataset, Dict):
-                self.sampler = {name: SequentialSampler(ds) for name, ds in dataset.items()}
-
-        elif isinstance(sampler, List):
-            if len(sampler) != len(dataset):
-                raise ValueError("the length of sampler != the length of sampler")
-            self.sampler = sampler
+            self.sampler = {name: SequentialSampler(ds) for name, ds in dataset.items()}
 
         elif isinstance(sampler, Dict):
             self.sampler = sampler
@@ -68,26 +63,7 @@ class MixSampler:
 
         # 计算扩展后的大数据集长度total_len和扩展后的单个数据集长度sampler_len
         sampler_lens, total_lens, sampler_index = [], 0, []
-        if isinstance(self.sampler, List):
-            if ds_ratio is None:
-                sampler_lens = [len(spl) for spl in self.sampler]
-
-            elif ds_ratio == 'pad_to_most':
-                sampler_lens = [max(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-            elif ds_ratio == 'truncate_to_least':
-                sampler_lens = [min(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-            elif isinstance(ds_ratio, List):
-                if not all(item >= 0 for item in ds_ratio):
-                    raise ValueError("batch_size should be a positive integer value, "
-                                     "but got ds_ratio={}".format(ds_ratio))
-                sampler_lens = [int(len(spl) * ratio) for spl, ratio in zip(self.sampler, ds_ratio)]
-            else:
-                raise ValueError(f"{ds_ratio} must be pad_to_least or truncate_to_least or None or List")
-            total_lens = sum(sampler_lens)
-
-        elif isinstance(self.sampler, Dict):
+        if isinstance(self.sampler, Dict):
             if ds_ratio is None:
                 sampler_lens = [len(spl) for _, spl in self.sampler.items()]
 
@@ -100,7 +76,7 @@ class MixSampler:
                 sampler_lens = [min(len(spl) for _, spl in self.sampler.items())] * sampler_len
 
             elif isinstance(ds_ratio, Dict):
-                if not all(item >= 0 for item in ds_ratio):
+                if not all([item >= 0 for item in ds_ratio.values()]):
                     raise ValueError("batch_size should be a positive integer value, "
                                      "but got ds_ratio={}".format(ds_ratio))
                 sampler_lens = [int(len(spl) * ds_ratio[name]) for name, spl in self.sampler.items()]
@@ -108,7 +84,7 @@ class MixSampler:
                 raise ValueError(f"{ds_ratio} must be pad_to_least or truncate_to_least or None or List")
             total_lens = sum(sampler_lens)
 
-        # sampler为str时候，初始化下移到iter方法中
+        # sampler 为 str 时候，初始化下移到 iter 方法中
         if len(sampler_lens) > 0:
             sampler_index = [sampler_lens[0]]
             for idx in sampler_lens[1:]:
@@ -160,75 +136,37 @@ class DopedSampler(MixSampler):
     """
     定制给MixDataLoader的BatchSampler，其功能是将传入的datasets的list列表混合采样组成一个个batch返回。
     """
-    def __init__(self, dataset: Union[List, Dict], batch_size: int = None,
-                 sampler: Union[List["Sampler"], Dict[str, "Sampler"], str] = None,
-                 ds_ratio: Union[str, None, List[float], Dict[str, float]] = None,
+    def __init__(self, dataset: Dict, batch_size: int = None,
+                 sampler: Union[Dict[str, "Sampler"], str] = None,
+                 ds_ratio: Union[str, None, Dict[str, float]] = None,
                  drop_last: bool = False, rank: int = -1, word_size: int = -1) -> None:
         super(DopedSampler, self).__init__(dataset=dataset, batch_size=batch_size,
                                            sampler=sampler, ds_ratio=ds_ratio,
                                            drop_last=drop_last, rank=rank, word_size=word_size)
 
     def __iter__(self) -> List[int]:
-        # sampler为str， 此时为单机多卡或者单机，可以实现rand随机化
+        # sampler 为 str， 此时为单机多卡或者单机，可以实现 rand 随机化
         if isinstance(self.sampler, str):
             if self.sampler == 'seq':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds))))
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds))))
             elif self.sampler == 'rand':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(indices[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(indices))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(indices)
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    g = torch.Generator()
+                    g.manual_seed(self.epoch)
+                    indices = torch.randperm(len(per_ds), generator=g).tolist()
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(indices)
 
             # 根据给定的ds_ratio计算真正需要处理数据集
-            if isinstance(self.sampler, List):
-                if self.ds_ratio is None:
-                    sampler_lens = [len(spl) for spl in self.sampler]
-
-                elif self.ds_ratio == 'pad_to_most':
-                    sampler_lens = [max(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-                elif self.ds_ratio == 'truncate_to_least':
-                    sampler_lens = [min(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-                elif isinstance(self.ds_ratio, List):
-                    if not all(item >= 0 for item in self.ds_ratio):
-                        raise ValueError("batch_size should be a positive integer value, "
-                                         "but got ds_ratio={}".format(self.ds_ratio))
-                    sampler_lens = [int(len(spl) * ratio) for spl, ratio in zip(self.sampler, self.ds_ratio)]
-                else:
-                    raise ValueError(f"{self.ds_ratio} must be pad_to_least or truncate_to_least or None or List")
-                total_lens = sum(sampler_lens)
-
-            elif isinstance(self.sampler, Dict):
+            if isinstance(self.sampler, Dict):
                 if self.ds_ratio is None:
                     sampler_lens = [len(spl) for _, spl in self.sampler.items()]
 
@@ -257,11 +195,11 @@ class DopedSampler(MixSampler):
                 sampler_index.append(temp + idx)
             self.num_samplers = sampler_index
             self.len_samplers = total_lens
-        # 每个batch的数据, 总的数据量total_index, 每个数据集的samplers
+        # 每个 batch 的数据, 总的数据量 total_index , 每个数据集的 samplers
         batch_idx, samplers = [], []
         # 如果单机则用所有数据，否则采用多卡
         if self.rank < 0 or self.word_size < 0:
-            # 根据sampler长度判断是否使用unsigned int 或者unsigned long
+            # 根据 sampler 长度判断是否使用 unsigned int 或者 unsigned long
             if self.len_samplers > 42e8:
                 total_index = array.array('L', list(range(self.len_samplers)))
             else:
@@ -274,15 +212,17 @@ class DopedSampler(MixSampler):
             else:
                 total_index = array.array('I', list(range(self.len_samplers))[self.rank::self.word_size])
 
+        start_idx = 0
+
+        # （特定数据集需要长度，特定数据集sampler, 特定数据集的基址， 特定sampler的下标）
+        for idx, (name, spl) in enumerate(self.sampler.items()):
+            end_idx = len(spl)
+            samplers.append((iter(spl), name, start_idx))
+            start_idx += end_idx
         # 根据sampler的类型取出每个数据集的sampler
-        if isinstance(self.sampler, List):
-            sampler_base_index = [0] + [len(spl) for spl in self.sampler][:-1]
-            samplers = [(iter(spl), idx, base_index)
-                        for idx, (spl, base_index) in enumerate(zip(self.sampler, sampler_base_index))]
-        else:
-            sampler_base_index = [0] + [len(spl) for _, spl in self.sampler.items()][:-1]
-            samplers = [(iter(spl), name, sampler_base_index[idx])
-                        for idx, (name, spl) in enumerate(self.sampler.items())]
+        # sampler_base_index = [0] + [len(spl) for _, spl in self.sampler.items()][:-1]
+        # samplers = [(iter(spl), name, sampler_base_index[idx])
+        #             for idx, (name, spl) in enumerate(self.sampler.items())]
         # 生成随机数
         np.random.seed(self.epoch)
         np.random.shuffle(total_index)
@@ -295,7 +235,7 @@ class DopedSampler(MixSampler):
                 # 重新初始化一个新的sampler，因为不可能为空，故一定不会出现stopIteration
                 spl = iter(self.sampler[name])
                 batch_idx.append(next(spl) + base_index)
-                samplers[name] = (spl, name, base_index)
+                samplers[ds_index] = (spl, name, base_index)
             if len(batch_idx) == self.batch_size:
                 yield batch_idx
                 batch_idx = []
@@ -343,63 +283,26 @@ class MixSequentialSampler(MixSampler):
         # sampler为str， 此时为单机多卡或者单机，可以实现rand随机化
         if isinstance(self.sampler, str):
             if self.sampler == 'seq':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds))))
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds))))
             elif self.sampler == 'rand':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(indices[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(indices))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(indices)
 
-            # 根据给定的ds_ratio计算真正需要处理数据集
-            if isinstance(self.sampler, List):
-                if self.ds_ratio is None:
-                    sampler_lens = [len(spl) for spl in self.sampler]
-
-                elif self.ds_ratio == 'pad_to_most':
-                    sampler_lens = [max(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-                elif self.ds_ratio == 'truncate_to_least':
-                    sampler_lens = [min(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-                elif isinstance(self.ds_ratio, List):
-                    if not all(item >= 0 for item in self.ds_ratio):
-                        raise ValueError("batch_size should be a positive integer value, "
-                                         "but got ds_ratio={}".format(self.ds_ratio))
-                    sampler_lens = [int(len(spl) * ratio) for spl, ratio in zip(self.sampler, self.ds_ratio)]
-                else:
-                    raise ValueError(f"{self.ds_ratio} must be pad_to_least or truncate_to_least or None or List")
-                total_lens = sum(sampler_lens)
-
-            elif isinstance(self.sampler, Dict):
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    g = torch.Generator()
+                    g.manual_seed(self.epoch)
+                    indices = torch.randperm(len(per_ds), generator=g).tolist()
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(indices)
+
+            # 根据给定的 ds_ratio 算真正需要处理数据集
+            if isinstance(self.sampler, Dict):
                 if self.ds_ratio is None:
                     sampler_lens = [len(spl) for _, spl in self.sampler.items()]
 
@@ -430,21 +333,20 @@ class MixSequentialSampler(MixSampler):
             self.len_samplers = total_lens
 
         batch_idx, total_index, samplers = [], list(range(self.len_samplers)), []
-        if isinstance(self.sampler, List):
-            if self.word_size > 0 and self.rank >= 0:
-                sampler_base_index = [0] + [len(spl) * self.word_size for spl in self.sampler][:-1]
-            else:
-                sampler_base_index = [0] + [len(spl) for spl in self.sampler][:-1]
-            samplers = [(iter(spl), idx, base_index) for idx, (spl, base_index) in
-                        enumerate(zip(self.sampler, sampler_base_index))]
-        else:
-            if self.word_size > 0 and self.rank >= 0:
-                sampler_base_index = [0] + [len(spl) * self.word_size for _, spl in self.sampler.items()][:-1]
-            else:
-                sampler_base_index = [0] + [len(spl) for _, spl in self.sampler.items()][:-1]
-
-            samplers = [(iter(spl), name, sampler_base_index[idx])
-                        for idx, (name, spl) in enumerate(self.sampler.items())]
+        start_idx = 0
+
+        # （特定数据集需要长度，特定数据集sampler, 特定数据集的基址， 特定sampler的下标）
+        for idx, (name, spl) in enumerate(self.sampler.items()):
+            end_idx = len(spl)
+            samplers.append((iter(spl), name, start_idx))
+            start_idx += end_idx
+        # if self.word_size > 0 and self.rank >= 0:
+        #     sampler_base_index = [0] + [len(spl) * self.word_size for _, spl in self.sampler.items()][:-1]
+        # else:
+        #     sampler_base_index = [0] + [len(spl) for _, spl in self.sampler.items()][:-1]
+        #
+        # samplers = [(iter(spl), name, sampler_base_index[idx])
+        #             for idx, (name, spl) in enumerate(self.sampler.items())]
         for idx in total_index:
             ds_index = np.searchsorted(self.num_samplers, idx, side='right')
 
@@ -455,7 +357,7 @@ class MixSequentialSampler(MixSampler):
                 # 重新初始化一个新的sampler，因为不可能为空，故一定不会出现stopIteration
                 spl = iter(self.sampler[name])
                 batch_idx.append(next(spl) + base_index)
-                samplers[name] = (spl, name, base_index)
+                samplers[ds_index] = (spl, name, base_index)
             if len(batch_idx) == self.batch_size:
                 yield batch_idx
                 batch_idx = []
@@ -506,63 +408,26 @@ class PollingSampler(MixSampler):
         # sampler为str， 此时为单机多卡或者单机，可以实现rand随机化
         if isinstance(self.sampler, str):
             if self.sampler == 'seq':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(list(range(len(per_ds)))))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(list(range(len(per_ds))))
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds)))[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(list(range(len(per_ds))))
             elif self.sampler == 'rand':
-                if isinstance(self.datasets, List):
-                    self.sampler = []
-                    for per_ds in self.datasets:
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler.append(InnerSampler(indices[self.rank::self.word_size]))
-                        else:
-                            self.sampler.append(InnerSampler(indices))
-                elif isinstance(self.datasets, Dict):
-                    self.sampler = {}
-                    for name, per_ds in self.datasets.items():
-                        g = torch.Generator()
-                        g.manual_seed(self.epoch)
-                        indices = torch.randperm(len(per_ds), generator=g).tolist()
-                        if self.word_size >= 0 and self.rank >= 0:
-                            self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
-                        else:
-                            self.sampler[name] = InnerSampler(indices)
 
-            # 根据给定的ds_ratio计算真正需要处理数据集
-            if isinstance(self.sampler, List):
-                if self.ds_ratio is None:
-                    sampler_lens = [len(spl) for spl in self.sampler]
-
-                elif self.ds_ratio == 'pad_to_most':
-                    sampler_lens = [max(len(spl) for spl in self.sampler)] * len(self.sampler)
+                self.sampler = {}
+                for name, per_ds in self.datasets.items():
+                    g = torch.Generator()
+                    g.manual_seed(self.epoch)
+                    indices = torch.randperm(len(per_ds), generator=g).tolist()
+                    if self.word_size >= 0 and self.rank >= 0:
+                        self.sampler[name] = InnerSampler(indices[self.rank::self.word_size])
+                    else:
+                        self.sampler[name] = InnerSampler(indices)
 
-                elif self.ds_ratio == 'truncate_to_least':
-                    sampler_lens = [min(len(spl) for spl in self.sampler)] * len(self.sampler)
-
-                elif isinstance(self.ds_ratio, List):
-                    if not all(item >= 0 for item in self.ds_ratio):
-                        raise ValueError("batch_size should be a positive integer value, "
-                                         "but got ds_ratio={}".format(self.ds_ratio))
-                    sampler_lens = [int(len(spl) * ratio) for spl, ratio in zip(self.sampler, self.ds_ratio)]
-                else:
-                    raise ValueError(f"{self.ds_ratio} must be pad_to_least or truncate_to_least or None or List")
-                total_lens = sum(sampler_lens)
-
-            elif isinstance(self.sampler, Dict):
+            # 根据给定的ds_ratio计算真正需要处理数据集
+            if isinstance(self.sampler, Dict):
                 if self.ds_ratio is None:
                     sampler_lens = [len(spl) for _, spl in self.sampler.items()]
 
@@ -592,17 +457,15 @@ class PollingSampler(MixSampler):
             self.num_samplers = sampler_index
             self.len_samplers = total_lens
 
-        start_idx, samplers = 0, []
-        if isinstance(self.sampler, List):
-            # （特定数据集需要长度，特定数据集sampler, 特定数据集的基址， 特定sampler的下标）
-            for sampler_idx, (end_idx, spl) in enumerate(zip(self.num_samplers, self.sampler)):
-                samplers.append((iter(range(start_idx, end_idx)), iter(spl), start_idx, sampler_idx))
-                start_idx = end_idx
-        else:
-            for idx, (name, spl) in enumerate(self.sampler.items()):
-                end_idx = self.num_samplers[idx]
-                samplers.append((iter(range(start_idx, end_idx)), iter(spl), start_idx, name))
-                start_idx = end_idx
+        start_idx, samplers, true_start_idx, true_end_idx = 0, [], 0, 0
+
+        # （特定数据集需要长度，特定数据集sampler, 特定数据集的基址， 特定sampler的下标）
+        for idx, (name, spl) in enumerate(self.sampler.items()):
+            end_idx = len(spl)
+            true_end_idx = self.num_samplers[idx]
+            samplers.append((iter(range(true_start_idx, true_end_idx)), iter(spl), start_idx, name))
+            start_idx += end_idx
+            true_start_idx = true_end_idx
 
         while True:
             # 退出循环
diff --git a/tests/core/dataloaders/test_mixdataloader.py b/tests/core/dataloaders/test_mixdataloader.py
new file mode 100644
index 00000000..35872b39
--- /dev/null
+++ b/tests/core/dataloaders/test_mixdataloader.py
@@ -0,0 +1,495 @@
+import pytest
+from typing import Mapping
+
+from fastNLP.core.dataloaders import MixDataLoader
+from fastNLP import DataSet
+from fastNLP.core.collators import Collator
+from fastNLP.envs.imports import _NEED_IMPORT_TORCH
+
+if _NEED_IMPORT_TORCH:
+    import torch
+    from torch.utils.data import default_collate, SequentialSampler, RandomSampler
+
+d1 = DataSet({"x": [[1, 2], [2, 3, 4], [4, 5, 6, 7]] * 10, "y": [1, 0, 1] * 10})
+
+d2 = DataSet({'x': [[101, 201], [201, 301, 401], [100]] * 10, 'y': [20, 10, 10] * 10})
+
+d3 = DataSet({'x': [[1000, 2000], [0], [2000, 3000, 4000, 5000]] * 100, 'y': [100, 100, 200] * 100})
+
+
+def test_pad_val(tensor, val=0):
+    if isinstance(tensor, torch.Tensor):
+        tensor = tensor.tolist()
+    for item in tensor:
+        if item[-1] > 0:
+            continue
+        elif item[-1] != val:
+            return False
+    return True
+
+
+class TestMixDataLoader:
+
+    def test_sequential_init(self):
+        datasets = {'d1': d1, 'd2': d2, 'd3': d3}
+        # drop_last = True, collate_fn = 'auto
+        dl = MixDataLoader(datasets=datasets, mode='sequential', collate_fn='auto', drop_last=True)
+        for idx, batch in enumerate(dl):
+            if idx == 0:
+                # d1
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                # d2
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx > 1:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+
+        # collate_fn = Callable
+        def collate_batch(batch):
+            new_batch = {'x': [], 'y': []}
+            for ins in batch:
+                new_batch['x'].append(ins['x'])
+                new_batch['y'].append(ins['y'])
+            return new_batch
+
+        dl1 = MixDataLoader(datasets=datasets, mode='sequential', collate_fn=collate_batch, drop_last=True)
+        for idx, batch in enumerate(dl1):
+            if idx == 0:
+                # d1
+                assert [1, 2] in batch['x']
+            if idx == 1:
+                # d2
+                assert [101, 201] in batch['x']
+            if idx > 1:
+                # d3
+                assert [1000, 2000] in batch['x']
+            assert 'x' in batch and 'y' in batch
+
+        collate_fns = {'d1': Collator(backend='auto').set_pad("x", -1),
+                       'd2': Collator(backend='auto').set_pad("x", -2),
+                       'd3': Collator(backend='auto').set_pad("x", -3)}
+        dl2 = MixDataLoader(datasets=datasets, mode='sequential', collate_fn=collate_fns, drop_last=True)
+        for idx, batch in enumerate(dl2):
+            if idx == 0:
+                assert test_pad_val(batch['x'], val=-1)
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                assert test_pad_val(batch['x'], val=-2)
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx > 1:
+                assert test_pad_val(batch['x'], val=-3)
+                assert batch['x'].shape == torch.Size([16, 4])
+
+        # sampler 为 str
+        dl3 = MixDataLoader(datasets=datasets, mode='sequential', sampler='seq', drop_last=True)
+        dl4 = MixDataLoader(datasets=datasets, mode='sequential', sampler='rand', drop_last=True)
+        for idx, batch in enumerate(dl3):
+            if idx == 0:
+                # d1
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx == 2:
+                # d3
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+            if idx > 1:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+
+        for idx, batch in enumerate(dl4):
+            if idx == 0:
+                # d1
+                assert batch['x'][:3].tolist() != [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                # d2
+                assert batch['x'][:3].tolist() != [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx == 2:
+                # d3
+                assert batch['x'][:3].tolist() != [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+            if idx > 1:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+
+        # sampler 为 Dict
+        samplers = {'d1': SequentialSampler(d1),
+                    'd2': SequentialSampler(d2),
+                    'd3': RandomSampler(d3)}
+        dl5 = MixDataLoader(datasets=datasets, mode='sequential', sampler=samplers, drop_last=True)
+        for idx, batch in enumerate(dl5):
+            if idx == 0:
+                # d1
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx > 1:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+
+        # ds_ratio 为 'truncate_to_least'
+        dl6 = MixDataLoader(datasets=datasets, mode='sequential', ds_ratio='truncate_to_least', drop_last=True)
+        for idx, batch in enumerate(dl6):
+            if idx == 0:
+                # d1
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape == torch.Size([16, 4])
+            if idx == 1:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape == torch.Size([16, 3])
+            if idx == 2:
+                # d3
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+            if idx > 2:
+                raise ValueError(f"ds_ratio: 'truncate_to_least' error")
+
+        # ds_ratio 为 'pad_to_most'
+        dl7 = MixDataLoader(datasets=datasets, mode='sequential', ds_ratio='pad_to_most', drop_last=True)
+        for idx, batch in enumerate(dl7):
+            if idx < 18:
+                # d1
+                assert batch['x'].shape == torch.Size([16, 4])
+            if 18 <= idx < 36:
+                # d2
+                assert batch['x'].shape == torch.Size([16, 3])
+            if 36 <= idx < 54:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 54:
+                raise ValueError(f"ds_ratio: 'pad_to_most' error")
+
+        # ds_ratio 为 Dict[str, float]
+        ds_ratio = {'d1': 1.0, 'd2': 2.0, 'd3': 2.0}
+        dl8 = MixDataLoader(datasets=datasets, mode='sequential', ds_ratio=ds_ratio, drop_last=True)
+        for idx, batch in enumerate(dl8):
+            if idx < 1:
+                # d1
+                assert batch['x'].shape == torch.Size([16, 4])
+            if 1 <= idx < 4:
+                # d2
+                assert batch['x'].shape == torch.Size([16, 3])
+            if 4 <= idx < 41:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 41:
+                raise ValueError(f"ds_ratio: 'pad_to_most' error")
+
+        ds_ratio = {'d1': 0.1, 'd2': 0.6, 'd3': 1.0}
+        dl9 = MixDataLoader(datasets=datasets, mode='sequential', ds_ratio=ds_ratio, drop_last=True)
+        for idx, batch in enumerate(dl9):
+            if idx < 1:
+                # d2
+                assert batch['x'].shape == torch.Size([16, 3])
+            if 1 <= idx < 19:
+                # d3
+                assert batch['x'].shape == torch.Size([16, 4])
+
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 19:
+                raise ValueError(f"ds_ratio: 'pad_to_most' error")
+
+    def test_mix(self):
+        datasets = {'d1': d1, 'd2': d2, 'd3': d3}
+        dl = MixDataLoader(datasets=datasets, mode='mix', collate_fn='auto', drop_last=True)
+        for idx, batch in enumerate(dl):
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 22:
+                raise ValueError(f"out of range")
+
+        # collate_fn = Callable
+        def collate_batch(batch):
+            new_batch = {'x': [], 'y': []}
+            for ins in batch:
+                new_batch['x'].append(ins['x'])
+                new_batch['y'].append(ins['y'])
+            return new_batch
+
+        dl1 = MixDataLoader(datasets=datasets, mode='mix', collate_fn=collate_batch, drop_last=True)
+        for idx, batch in enumerate(dl1):
+            assert isinstance(batch['x'], list)
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 22:
+                raise ValueError(f"out of range")
+
+        collate_fns = {'d1': Collator(backend='auto').set_pad("x", -1),
+                       'd2': Collator(backend='auto').set_pad("x", -2),
+                       'd3': Collator(backend='auto').set_pad("x", -3)}
+        with pytest.raises(ValueError):
+            MixDataLoader(datasets=datasets, mode='mix', collate_fn=collate_fns)
+
+        # sampler 为 str
+        dl3 = MixDataLoader(datasets=datasets, mode='mix', sampler='seq', drop_last=True)
+        for idx, batch in enumerate(dl3):
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 22:
+                raise ValueError(f"out of range")
+        dl4 = MixDataLoader(datasets=datasets, mode='mix', sampler='rand', drop_last=True)
+        for idx, batch in enumerate(dl4):
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 22:
+                raise ValueError(f"out of range")
+        # sampler 为 Dict
+        samplers = {'d1': SequentialSampler(d1),
+                    'd2': SequentialSampler(d2),
+                    'd3': RandomSampler(d3)}
+        dl5 = MixDataLoader(datasets=datasets, mode='mix', sampler=samplers, drop_last=True)
+        for idx, batch in enumerate(dl5):
+            assert test_pad_val(batch['x'], val=0)
+            if idx >= 22:
+                raise ValueError(f"out of range")
+        # ds_ratio 为 'truncate_to_least'
+        dl6 = MixDataLoader(datasets=datasets, mode='mix', ds_ratio='truncate_to_least')
+        d1_len, d2_len, d3_len = 0, 0, 0
+        for idx, batch in enumerate(dl6):
+            for item in batch['y'].tolist():
+                if item in [1, 0, 1]:
+                    d1_len += 1
+                elif item in [20, 10, 10]:
+                    d2_len += 1
+                elif item in [100, 100, 200]:
+                    d3_len += 1
+            if idx >= 6:
+                raise ValueError(f"ds_ratio 为 'truncate_to_least'出错了")
+        assert d1_len == d2_len == d3_len == 30
+
+        # ds_ratio 为 'pad_to_most'
+        dl7 = MixDataLoader(datasets=datasets, mode='mix', ds_ratio='pad_to_most')
+        d1_len, d2_len, d3_len = 0, 0, 0
+        for idx, batch in enumerate(dl7):
+            for item in batch['y'].tolist():
+                if item in [1, 0, 1]:
+                    d1_len += 1
+                elif item in [20, 10, 10]:
+                    d2_len += 1
+                elif item in [100, 100, 200]:
+                    d3_len += 1
+
+            if idx >= 57:
+                raise ValueError(f"ds_ratio 为 'pad_to_most'出错了")
+        assert d1_len == d2_len == d3_len == 300
+
+        # ds_ratio 为 Dict[str, float]
+        ds_ratio = {'d1': 1.0, 'd2': 2.0, 'd3': 2.0}
+        dl8 = MixDataLoader(datasets=datasets, mode='mix', ds_ratio=ds_ratio)
+        d1_len, d2_len, d3_len = 0, 0, 0
+        for idx, batch in enumerate(dl8):
+            for item in batch['y'].tolist():
+                if item in [1, 0, 1]:
+                    d1_len += 1
+                elif item in [20, 10, 10]:
+                    d2_len += 1
+                elif item in [100, 100, 200]:
+                    d3_len += 1
+            if idx >= 44:
+                raise ValueError(f"ds_ratio 为 'Dict'出错了")
+        assert d1_len == 30
+        assert d2_len == 60
+        assert d3_len == 600
+
+        ds_ratio = {'d1': 0.1, 'd2': 0.6, 'd3': 1.0}
+        dl9 = MixDataLoader(datasets=datasets, mode='mix', ds_ratio=ds_ratio)
+        d1_len, d2_len, d3_len = 0, 0, 0
+        for idx, batch in enumerate(dl9):
+            for item in batch['y'].tolist():
+                if item in [1, 0, 1]:
+                    d1_len += 1
+                elif item in [20, 10, 10]:
+                    d2_len += 1
+                elif item in [100, 100, 200]:
+                    d3_len += 1
+            if idx >= 21:
+                raise ValueError(f"ds_ratio 为 'Dict'出错了")
+
+    def test_polling(self):
+        datasets = {'d1': d1, 'd2': d2, 'd3': d3}
+        dl = MixDataLoader(datasets=datasets, mode='polling', collate_fn='auto', batch_size=18)
+        for idx, batch in enumerate(dl):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or 4 < idx <= 20:
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx > 20:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+
+        # collate_fn = Callable
+        def collate_batch(batch):
+            new_batch = {'x': [], 'y': []}
+            for ins in batch:
+                new_batch['x'].append(ins['x'])
+                new_batch['y'].append(ins['y'])
+            return new_batch
+
+        dl1 = MixDataLoader(datasets=datasets, mode='polling', collate_fn=collate_batch, batch_size=18)
+        for idx, batch in enumerate(dl1):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3] == [[1, 2], [2, 3, 4], [4, 5, 6, 7]]
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'][:3] == [[101, 201], [201, 301, 401], [100]]
+            elif idx == 2 or 4 < idx <= 20:
+                assert batch['x'][:3] == [[1000, 2000], [0], [2000, 3000, 4000, 5000]]
+            if idx > 20:
+                raise ValueError(f"out of range")
+
+        collate_fns = {'d1': Collator(backend='auto').set_pad("x", -1),
+                       'd2': Collator(backend='auto').set_pad("x", -2),
+                       'd3': Collator(backend='auto').set_pad("x", -3)}
+        dl1 = MixDataLoader(datasets=datasets, mode='polling', collate_fn=collate_fns, batch_size=18)
+        for idx, batch in enumerate(dl1):
+            if idx == 0 or idx == 3:
+                assert test_pad_val(batch['x'], val=-1)
+                assert batch['x'][:3].tolist() == [[1, 2, -1, -1], [2, 3, 4, -1], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert test_pad_val(batch['x'], val=-2)
+                assert batch['x'][:3].tolist() == [[101, 201, -2], [201, 301, 401], [100, -2, -2]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or 4 < idx <= 20:
+                assert test_pad_val(batch['x'], val=-3)
+                assert batch['x'][:3].tolist() == [[1000, 2000, -3, -3], [0, -3, -3, -3], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx > 20:
+                raise ValueError(f"out of range")
+
+        # sampler 为 str
+        dl2 = MixDataLoader(datasets=datasets, mode='polling', sampler='seq', batch_size=18)
+        dl3 = MixDataLoader(datasets=datasets, mode='polling', sampler='rand', batch_size=18)
+        for idx, batch in enumerate(dl2):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or 4 < idx <= 20:
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx > 20:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+        for idx, batch in enumerate(dl3):
+            if idx == 0 or idx == 3:
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or 4 < idx <= 20:
+                assert batch['x'].shape[1] == 4
+            if idx > 20:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+        # sampler 为 Dict
+        samplers = {'d1': SequentialSampler(d1),
+                    'd2': SequentialSampler(d2),
+                    'd3': RandomSampler(d3)}
+        dl4 = MixDataLoader(datasets=datasets, mode='polling', sampler=samplers, batch_size=18)
+        for idx, batch in enumerate(dl4):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or 4 < idx <= 20:
+                assert batch['x'].shape[1] == 4
+            if idx > 20:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+
+        # ds_ratio 为 'truncate_to_least'
+        dl5 = MixDataLoader(datasets=datasets, mode='polling', ds_ratio='truncate_to_least', batch_size=18)
+        for idx, batch in enumerate(dl5):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or idx == 5:
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx > 5:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+
+        # ds_ratio 为 'pad_to_most'
+        dl6 = MixDataLoader(datasets=datasets, mode='polling', ds_ratio='pad_to_most', batch_size=18)
+        for idx, batch in enumerate(dl6):
+            if idx % 3 == 0:
+                # d1
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            if idx % 3 == 1:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            if idx % 3 == 2:
+                # d3
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx >= 51:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+
+        # ds_ratio 为 Dict[str, float]
+        ds_ratio = {'d1': 1.0, 'd2': 2.0, 'd3': 2.0}
+        dl7 = MixDataLoader(datasets=datasets, mode='polling', ds_ratio=ds_ratio, batch_size=18)
+        for idx, batch in enumerate(dl7):
+            if idx == 0 or idx == 3:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1 or idx == 4 or idx == 6 or idx == 8:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx == 2 or idx == 5 or idx == 7 or idx > 8:
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+            if idx > 39:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
+
+        ds_ratio = {'d1': 0.1, 'd2': 0.6, 'd3': 1.0}
+        dl8 = MixDataLoader(datasets=datasets, mode='polling', ds_ratio=ds_ratio, batch_size=18)
+        for idx, batch in enumerate(dl8):
+            if idx == 0:
+                assert batch['x'][:3].tolist() == [[1, 2, 0, 0], [2, 3, 4, 0], [4, 5, 6, 7]]
+                assert batch['x'].shape[1] == 4
+            elif idx == 1:
+                # d2
+                assert batch['x'][:3].tolist() == [[101, 201, 0], [201, 301, 401], [100, 0, 0]]
+                assert batch['x'].shape[1] == 3
+            elif idx > 1:
+                assert batch['x'][:3].tolist() == [[1000, 2000, 0, 0], [0, 0, 0, 0], [2000, 3000, 4000, 5000]]
+                assert batch['x'].shape[1] == 4
+
+            if idx > 18:
+                raise ValueError(f"out of range")
+            test_pad_val(batch['x'], val=0)
\ No newline at end of file