diff --git a/fastNLP/core/dataset.py b/fastNLP/core/dataset.py
index 76a34655..f0e27b83 100644
--- a/fastNLP/core/dataset.py
+++ b/fastNLP/core/dataset.py
@@ -151,16 +151,19 @@ class DataSet(object):
                 assert name in self.field_arrays
                 self.field_arrays[name].append(field)
 
-    def add_field(self, name, fields, padder=AutoPadder(pad_val=0), is_input=False, is_target=False, ignore_type=False):
+    def add_field(self, name, fields, padder=None, is_input=False, is_target=False, ignore_type=False):
         """Add a new field to the DataSet.
         
         :param str name: the name of the field.
         :param fields: a list of int, float, or other objects.
-        :param int padder: PadBase对象，如何对该Field进行padding。大部分情况使用默认值即可
+        :param padder: PadBase对象，如何对该Field进行padding。如果为None则使用
         :param bool is_input: whether this field is model input.
         :param bool is_target: whether this field is label or target.
         :param bool ignore_type: If True, do not perform type check. (Default: False)
         """
+        if padder is None:
+            padder = AutoPadder(pad_val=0)
+
         if len(self.field_arrays) != 0:
             if len(self) != len(fields):
                 raise RuntimeError(f"The field to append must have the same size as dataset. "
@@ -231,8 +234,8 @@ class DataSet(object):
                 raise KeyError("{} is not a valid field name.".format(name))
 
     def set_padder(self, field_name, padder):
-        """
-        为field_name设置padder
+        """为field_name设置padder
+
         :param field_name: str, 设置field的padding方式为padder
         :param padder: PadderBase类型或None. 设置为None即删除padder。即对该field不进行padding操作.
         :return:
@@ -242,8 +245,7 @@ class DataSet(object):
         self.field_arrays[field_name].set_padder(padder)
 
     def set_pad_val(self, field_name, pad_val):
-        """
-        为某个
+        """为某个field设置对应的pad_val.
 
         :param field_name: str，修改该field的pad_val
         :param pad_val: int，该field的padder会以pad_val作为padding index
@@ -254,43 +256,60 @@ class DataSet(object):
         self.field_arrays[field_name].set_pad_val(pad_val)
 
     def get_input_name(self):
-        """Get all field names with `is_input` as True.
+        """返回所有is_input被设置为True的field名称
 
-        :return field_names: a list of str
+        :return list, 里面的元素为被设置为input的field名称
         """
         return [name for name, field in self.field_arrays.items() if field.is_input]
 
     def get_target_name(self):
-        """Get all field names with `is_target` as True.
+        """返回所有is_target被设置为True的field名称
 
-        :return field_names: a list of str
+        :return list, 里面的元素为被设置为target的field名称
         """
         return [name for name, field in self.field_arrays.items() if field.is_target]
 
-    def apply(self, func, new_field_name=None, **kwargs):
-        """Apply a function to every instance of the DataSet.
-
-        :param func: a function that takes an instance as input.
-        :param str new_field_name: If not None, results of the function will be stored as a new field.
-        :param **kwargs: Accept parameters will be
-            (1) is_input: boolean, will be ignored if new_field is None. If True, the new field will be as input.
-            (2) is_target: boolean, will be ignored if new_field is None. If True, the new field will be as target.
-        :return results: if new_field_name is not passed, returned values of the function over all instances.
+    def apply_field(self, func, field_name, new_field_name=None, **kwargs):
+        """将DataSet中的每个instance中的`field_name`这个field传给func，并获取它的返回值.
+
+        :param func: Callable, input是instance的`field_name`这个field.
+        :param field_name: str, 传入func的是哪个field.
+        :param new_field_name: (str, None). 如果不是None，将func的返回值放入这个名为`new_field_name`的新field中，如果名称与已有
+                               的field相同，则覆盖之前的field.
+        :param **kwargs: 合法的参数有以下三个
+                        (1) is_input: bool, 如果为True则将`new_field_name`这个field设置为input
+                        (2) is_target: bool, 如果为True则将`new_field_name`这个field设置为target
+                        (3) ignore_type: bool, 如果为True则将`new_field_name`这个field的ignore_type设置为true, 忽略其类型
+        :return: List[], 里面的元素为func的返回值，所以list长度为DataSet的长度
         """
-        assert len(self)!=0, "Null dataset cannot use .apply()."
+        assert len(self)!=0, "Null DataSet cannot use apply()."
+        if field_name not in self:
+            raise KeyError("DataSet has no field named `{}`.".format(field_name))
         results = []
         idx = -1
         try:
             for idx, ins in enumerate(self._inner_iter()):
-                results.append(func(ins))
+                results.append(func(ins[field_name]))
         except Exception as e:
             if idx!=-1:
                 print("Exception happens at the `{}`th instance.".format(idx))
             raise e
-        # results = [func(ins) for ins in self._inner_iter()]
         if not (new_field_name is None) and len(list(filter(lambda x: x is not None, results))) == 0:  # all None
             raise ValueError("{} always return None.".format(get_func_signature(func=func)))
 
+        if new_field_name is not None:
+            self._add_apply_field(results, new_field_name, kwargs)
+
+        return results
+
+    def _add_apply_field(self, results, new_field_name, kwargs):
+        """将results作为加入到新的field中，field名称为new_field_name
+
+        :param results: List[], 一般是apply*()之后的结果
+        :param new_field_name: str, 新加入的field的名称
+        :param kwargs: dict, 用户apply*()时传入的自定义参数
+        :return:
+        """
         extra_param = {}
         if 'is_input' in kwargs:
             extra_param['is_input'] = kwargs['is_input']
@@ -298,56 +317,84 @@ class DataSet(object):
             extra_param['is_target'] = kwargs['is_target']
         if 'ignore_type' in kwargs:
             extra_param['ignore_type'] = kwargs['ignore_type']
-        if new_field_name is not None:
-            if new_field_name in self.field_arrays:
-                # overwrite the field, keep same attributes
-                old_field = self.field_arrays[new_field_name]
-                if 'is_input' not in extra_param:
-                    extra_param['is_input'] = old_field.is_input
-                if 'is_target' not in extra_param:
-                    extra_param['is_target'] = old_field.is_target
-                if 'ignore_type' not in extra_param:
-                    extra_param['ignore_type'] = old_field.ignore_type
-                self.add_field(name=new_field_name, fields=results, is_input=extra_param["is_input"],
-                               is_target=extra_param["is_target"], ignore_type=extra_param['ignore_type'])
-            else:
-                self.add_field(name=new_field_name, fields=results, is_input=extra_param.get("is_input", None),
-                               is_target=extra_param.get("is_target", None),
-                               ignore_type=extra_param.get("ignore_type", False))
+        if new_field_name in self.field_arrays:
+            # overwrite the field, keep same attributes
+            old_field = self.field_arrays[new_field_name]
+            if 'is_input' not in extra_param:
+                extra_param['is_input'] = old_field.is_input
+            if 'is_target' not in extra_param:
+                extra_param['is_target'] = old_field.is_target
+            if 'ignore_type' not in extra_param:
+                extra_param['ignore_type'] = old_field.ignore_type
+            self.add_field(name=new_field_name, fields=results, is_input=extra_param["is_input"],
+                           is_target=extra_param["is_target"], ignore_type=extra_param['ignore_type'])
         else:
-            return results
+            self.add_field(name=new_field_name, fields=results, is_input=extra_param.get("is_input", None),
+                           is_target=extra_param.get("is_target", None),
+                           ignore_type=extra_param.get("ignore_type", False))
+
+    def apply(self, func, new_field_name=None, **kwargs):
+        """将DataSet中每个instance传入到func中，并获取它的返回值.
+
+        :param func: Callable, 参数是DataSet中的instance
+        :param new_field_name: (None, str). (1) None, 不创建新的field; (2) str，将func的返回值放入这个名为
+                              `new_field_name`的新field中，如果名称与已有的field相同，则覆盖之前的field;
+        :param kwargs: 合法的参数有以下三个
+                        (1) is_input: bool, 如果为True则将`new_field_name`的field设置为input
+                        (2) is_target: bool, 如果为True则将`new_field_name`的field设置为target
+                        (3) ignore_type: bool, 如果为True则将`new_field_name`的field的ignore_type设置为true, 忽略其类型
+        :return: List[], 里面的元素为func的返回值，所以list长度为DataSet的长度
+        """
+        assert len(self)!=0, "Null DataSet cannot use apply()."
+        idx = -1
+        try:
+            results = []
+            for idx, ins in enumerate(self._inner_iter()):
+                results.append(func(ins))
+        except Exception as e:
+            if idx!=-1:
+                print("Exception happens at the `{}`th instance.".format(idx))
+            raise e
+        # results = [func(ins) for ins in self._inner_iter()]
+        if not (new_field_name is None) and len(list(filter(lambda x: x is not None, results))) == 0:  # all None
+            raise ValueError("{} always return None.".format(get_func_signature(func=func)))
+
+        if new_field_name is not None:
+            self._add_apply_field(results, new_field_name, kwargs)
+
+        return results
 
     def drop(self, func, inplace=True):
-        """Drop instances if a condition holds.
+        """func接受一个instance，返回bool值，返回值为True时，该instance会被删除。
 
-        :param func: a function that takes an Instance object as input, and returns bool.
-            The instance will be dropped if the function returns True.
-        :param inplace: bool, whether to drop inpalce. Otherwise a new dataset will be returned.
+        :param func: Callable, 接受一个instance作为参数，返回bool值。为True时删除该instance
+        :param inplace: bool, 是否在当前DataSet中直接删除instance。如果为False，返回值为一个删除了相应instance的新的DataSet
 
+        :return: DataSet.
         """
         if inplace:
             results = [ins for ins in self._inner_iter() if not func(ins)]
             for name, old_field in self.field_arrays.items():
                 self.field_arrays[name].content = [ins[name] for ins in results]
+            return self
         else:
             results = [ins for ins in self if not func(ins)]
             data = DataSet(results)
             for field_name, field in self.field_arrays.items():
                 data.field_arrays[field_name].to(field)
+            return data
 
-    def split(self, dev_ratio):
-        """Split the dataset into training and development(validation) set.
+    def split(self, ratio):
+        """将DataSet按照ratio的比例拆分，返回两个DataSet
 
-        :param float dev_ratio: the ratio of test set in all data.
-        :return (train_set, dev_set):
-                train_set: the training set
-                dev_set: the development set
+        :param ratio: float, 0<ratio<1, 返回的第一个DataSet拥有ratio这么多数据，第二个DataSet拥有(1-ratio)这么多数据
+        :return (DataSet, DataSet)
         """
-        assert isinstance(dev_ratio, float)
-        assert 0 < dev_ratio < 1
+        assert isinstance(ratio, float)
+        assert 0 < ratio < 1
         all_indices = [_ for _ in range(len(self))]
         np.random.shuffle(all_indices)
-        split = int(dev_ratio * len(self))
+        split = int(ratio * len(self))
         dev_indices = all_indices[:split]
         train_indices = all_indices[split:]
         dev_set = DataSet()
@@ -398,26 +445,25 @@ class DataSet(object):
                     _dict[header].append(content)
         return cls(_dict)
 
-    # def read_pos(self):
-    #     return DataLoaderRegister.get_reader('read_pos')
-
     def save(self, path):
-        """Save the DataSet object as pickle.
+        """保存DataSet.
 
-        :param str path: the path to the pickle
+        :param path: str, 将DataSet存在哪个路径
         """
         with open(path, 'wb') as f:
             pickle.dump(self, f)
 
     @staticmethod
     def load(path):
-        """Load a DataSet object from pickle.
+        """从保存的DataSet pickle路径中读取DataSet
 
-        :param str path: the path to the pickle
-        :return data_set:
+        :param path: str, 读取路径
+        :return  DataSet:
         """
         with open(path, 'rb') as f:
-            return pickle.load(f)
+            d = pickle.load(f)
+            assert isinstance(d, DataSet), "The object is not DataSet, but {}.".format(type(d))
+        return d
 
 
 def construct_dataset(sentences):
diff --git a/fastNLP/core/fieldarray.py b/fastNLP/core/fieldarray.py
index 10fbbebe..fa62541e 100644
--- a/fastNLP/core/fieldarray.py
+++ b/fastNLP/core/fieldarray.py
@@ -84,7 +84,7 @@ class AutoPadder(PadderBase):
             for i, content in enumerate(contents):
                 array[i][:len(content)] = content
         elif field_ele_dtype is None:
-            array = contents  # 当ignore_type=True时，直接返回contents
+            array = np.array(contents)  # 当ignore_type=True时，直接返回contents
         else:  # should only be str
             array = np.array([content for content in contents])
         return array
@@ -290,9 +290,10 @@ class FieldArray(object):
         return "FieldArray {}: {}".format(self.name, self.content.__repr__())
 
     def append(self, val):
-        """Add a new item to the tail of FieldArray.
+        """将val增加到FieldArray中，若该field的ignore_type为True则直接append到这个field中；若ignore_type为False，且当前field为
+        input或者target，则会检查传入的content是否与之前的内容在dimension, 元素的类型上是匹配的。
 
-        :param val: int, float, str, or a list of one.
+        :param val: Any.
         """
         if self.ignore_type is False:
             if isinstance(val, list):
@@ -367,13 +368,14 @@ class FieldArray(object):
         self.padder = deepcopy(padder)
 
     def set_pad_val(self, pad_val):
-        """
-        修改padder的pad_val.
-        :param pad_val: int。
+        """修改padder的pad_val.
+
+        :param pad_val: int。将该field的pad值设置为该值
         :return:
         """
         if self.padder is not None:
             self.padder.set_pad_val(pad_val)
+        return self
 
 
     def __len__(self):
@@ -385,8 +387,7 @@ class FieldArray(object):
 
     def to(self, other):
         """
-        将other的属性复制给本fieldarray(必须通过fieldarray类型). 包含 is_input, is_target, padder, dtype, pytype, content_dim
-            ignore_type
+        将other的属性复制给本FieldArray(other必须为FieldArray类型). 包含 is_input, is_target, padder, ignore_type
 
         :param other: FieldArray
         :return:
@@ -396,11 +397,10 @@ class FieldArray(object):
         self.is_input = other.is_input
         self.is_target = other.is_target
         self.padder = other.padder
-        self.dtype = other.dtype
-        self.pytype = other.pytype
-        self.content_dim = other.content_dim
         self.ignore_type = other.ignore_type
 
+        return self
+
 def is_iterable(content):
     try:
         _ = (e for e in content)
diff --git a/fastNLP/core/utils.py b/fastNLP/core/utils.py
index d9141412..1b2293ba 100644
--- a/fastNLP/core/utils.py
+++ b/fastNLP/core/utils.py
@@ -24,7 +24,7 @@ def _prepare_cache_filepath(filepath):
     if not os.path.exists(cache_dir):
         os.makedirs(cache_dir)
 
-
+#  TODO 可以保存下缓存时的参数，如果load的时候发现参数不一致，发出警告。
 def cache_results(cache_filepath, refresh=False, verbose=1):
     def wrapper_(func):
         signature = inspect.signature(func)
diff --git a/fastNLP/models/sequence_modeling.py b/fastNLP/models/sequence_modeling.py
index cb9e9478..cb615daf 100644
--- a/fastNLP/models/sequence_modeling.py
+++ b/fastNLP/models/sequence_modeling.py
@@ -79,7 +79,7 @@ class SeqLabeling(BaseModel):
         :return prediction: list of [decode path(list)]
         """
         max_len = x.shape[1]
-        tag_seq = self.Crf.viterbi_decode(x, self.mask)
+        tag_seq, _ = self.Crf.viterbi_decode(x, self.mask)
         # pad prediction to equal length
         if pad is True:
             for pred in tag_seq:
diff --git a/fastNLP/modules/decoder/CRF.py b/fastNLP/modules/decoder/CRF.py
index df004224..99e7a9c2 100644
--- a/fastNLP/modules/decoder/CRF.py
+++ b/fastNLP/modules/decoder/CRF.py
@@ -2,12 +2,7 @@ import torch
 from torch import nn
 
 from fastNLP.modules.utils import initial_parameter
-
-
-def log_sum_exp(x, dim=-1):
-    max_value, _ = x.max(dim=dim, keepdim=True)
-    res = torch.log(torch.sum(torch.exp(x - max_value), dim=dim, keepdim=True)) + max_value
-    return res.squeeze(dim)
+from fastNLP.modules.decoder.utils import log_sum_exp
 
 
 def seq_len_to_byte_mask(seq_lens):
@@ -20,22 +15,27 @@ def seq_len_to_byte_mask(seq_lens):
     return mask
 
 
-def allowed_transitions(id2label, encoding_type='bio'):
+def allowed_transitions(id2label, encoding_type='bio', include_start_end=True):
     """
+    给定一个id到label的映射表，返回所有可以跳转的(from_tag_id, to_tag_id)列表。
 
-    :param dict id2label: key是label的indices，value是str类型的tag或tag-label。value可以是只有tag的, 比如"B", "M"; 也可以是
-        "B-NN", "M-NN", tag和label之间一定要用"-"隔开。一般可以通过Vocabulary.get_id2word()id2label。
+    :param id2label: Dict, key是label的indices，value是str类型的tag或tag-label。value可以是只有tag的, 比如"B", "M"; 也可以是
+        "B-NN", "M-NN", tag和label之间一定要用"-"隔开。一般可以通过Vocabulary.get_id2word()得到id2label。
     :param encoding_type: str, 支持"bio", "bmes", "bmeso"。
-    :return: List[Tuple(int, int)]], 内部的Tuple是(from_tag_id, to_tag_id)。 返回的结果考虑了start和end，比如"BIO"中，B、O可以
-        位于序列的开端，而I不行。所以返回的结果中会包含(start_idx, B_idx), (start_idx, O_idx), 但是不包含(start_idx, I_idx).
-        start_idx=len(id2label), end_idx=len(id2label)+1。
+    :param include_start_end: bool, 是否包含开始与结尾的转换。比如在bio中，b/o可以在开头，但是i不能在开头；
+        为True，返回的结果中会包含(start_idx, b_idx), (start_idx, o_idx), 但是不包含(start_idx, i_idx);
+            start_idx=len(id2label), end_idx=len(id2label)+1。
+        为False, 返回的结果中不含与开始结尾相关的内容
+    :return: List[Tuple(int, int)]], 内部的Tuple是可以进行跳转的(from_tag_id, to_tag_id)。
     """
     num_tags = len(id2label)
     start_idx = num_tags
     end_idx = num_tags + 1
     encoding_type = encoding_type.lower()
     allowed_trans = []
-    id_label_lst = list(id2label.items()) + [(start_idx, 'start'), (end_idx, 'end')]
+    id_label_lst = list(id2label.items())
+    if include_start_end:
+        id_label_lst += [(start_idx, 'start'), (end_idx, 'end')]
     def split_tag_label(from_label):
         from_label = from_label.lower()
         if from_label in ['start', 'end']:
@@ -54,12 +54,12 @@ def allowed_transitions(id2label, encoding_type='bio'):
             if to_label in ['<pad>', '<unk>']:
                 continue
             to_tag, to_label = split_tag_label(to_label)
-            if is_transition_allowed(encoding_type, from_tag, from_label, to_tag, to_label):
+            if _is_transition_allowed(encoding_type, from_tag, from_label, to_tag, to_label):
                 allowed_trans.append((from_id, to_id))
     return allowed_trans
 
 
-def is_transition_allowed(encoding_type, from_tag, from_label, to_tag, to_label):
+def _is_transition_allowed(encoding_type, from_tag, from_label, to_tag, to_label):
     """
 
     :param encoding_type: str, 支持"BIO", "BMES", "BEMSO"。
@@ -140,20 +140,22 @@ def is_transition_allowed(encoding_type, from_tag, from_label, to_tag, to_label)
             raise ValueError("Unexpect tag type {}. Expect only 'B', 'M', 'E', 'S', 'O'.".format(from_tag))
 
     else:
-        raise ValueError("Only support BIO, BMES encoding type, got {}.".format(encoding_type))
+        raise ValueError("Only support BIO, BMES, BMESO encoding type, got {}.".format(encoding_type))
 
 
 class ConditionalRandomField(nn.Module):
-    """
-
-    :param int num_tags: 标签的数量。
-    :param bool include_start_end_trans: 是否包含起始tag
-    :param list allowed_transitions: ``List[Tuple[from_tag_id(int), to_tag_id(int)]]``. 允许的跃迁，可以通过allowed_transitions()得到。
-        如果为None，则所有跃迁均为合法
-    :param str initial_method:
-    """
-
-    def __init__(self, num_tags, include_start_end_trans=False, allowed_transitions=None, initial_method=None):
+    def __init__(self, num_tags, include_start_end_trans=False, allowed_transitions=None,
+                 initial_method=None):
+        """条件随机场。
+        提供forward()以及viterbi_decode()两个方法，分别用于训练与inference。
+
+        :param num_tags: int, 标签的数量
+        :param include_start_end_trans: bool, 是否考虑各个tag作为开始以及结尾的分数。
+        :param allowed_transitions: List[Tuple[from_tag_id(int), to_tag_id(int)]], 内部的Tuple[from_tag_id(int),
+                                   to_tag_id(int)]视为允许发生的跃迁，其他没有包含的跃迁认为是禁止跃迁，可以通过
+                                   allowed_transitions()函数得到；如果为None，则所有跃迁均为合法
+        :param initial_method: str, 初始化方法。见initial_parameter
+        """
         super(ConditionalRandomField, self).__init__()
 
         self.include_start_end_trans = include_start_end_trans
@@ -168,18 +170,12 @@ class ConditionalRandomField(nn.Module):
         if allowed_transitions is None:
             constrain = torch.zeros(num_tags + 2, num_tags + 2)
         else:
-            constrain = torch.ones(num_tags + 2, num_tags + 2) * -1000
+            constrain = torch.new_full((num_tags+2, num_tags+2), fill_value=-10000.0, dtype=torch.float)
             for from_tag_id, to_tag_id in allowed_transitions:
                 constrain[from_tag_id, to_tag_id] = 0
         self._constrain = nn.Parameter(constrain, requires_grad=False)
 
-        # self.reset_parameter()
         initial_parameter(self, initial_method)
-    def reset_parameter(self):
-        nn.init.xavier_normal_(self.trans_m)
-        if self.include_start_end_trans:
-            nn.init.normal_(self.start_scores)
-            nn.init.normal_(self.end_scores)
 
     def _normalizer_likelihood(self, logits, mask):
         """Computes the (batch_size,) denominator term for the log-likelihood, which is the
@@ -239,10 +235,11 @@ class ConditionalRandomField(nn.Module):
 
     def forward(self, feats, tags, mask):
         """
-        Calculate the neg log likelihood
-        :param feats:FloatTensor, batch_size x max_len x num_tags
-        :param tags:LongTensor, batch_size x max_len
-        :param mask:ByteTensor batch_size x max_len
+        用于计算CRF的前向loss，返回值为一个batch_size的FloatTensor，可能需要mean()求得loss。
+
+        :param feats:FloatTensor, batch_size x max_len x num_tags，特征矩阵。
+        :param tags:LongTensor, batch_size x max_len，标签矩阵。
+        :param mask:ByteTensor batch_size x max_len，为0的位置认为是padding。
         :return:FloatTensor, batch_size
         """
         feats = feats.transpose(0, 1)
@@ -253,28 +250,27 @@ class ConditionalRandomField(nn.Module):
 
         return all_path_score - gold_path_score
 
-    def viterbi_decode(self, data, mask, get_score=False, unpad=False):
-        """Given a feats matrix, return best decode path and best score.
+    def viterbi_decode(self, feats, mask, unpad=False):
+        """给定一个特征矩阵以及转移分数矩阵，计算出最佳的路径以及对应的分数
 
-        :param data:FloatTensor, batch_size x max_len x num_tags
-        :param mask:ByteTensor batch_size x max_len
-        :param get_score: bool, whether to output the decode score.
-        :param unpad: bool, 是否将结果unpad,
-                            如果False, 返回的是batch_size x max_len的tensor，
-                            如果True，返回的是List[List[int]], List[int]为每个sequence的label，已经unpadding了，即每个
-                                List[int]的长度是这个sample的有效长度
-        :return: 如果get_score为False，返回结果根据unpadding变动
-                 如果get_score为True, 返回 (paths, List[float], )。第一个仍然是解码后的路径(根据unpad变化)，第二个List[Float]
-                    为每个seqence的解码分数。
+        :param feats: FloatTensor, batch_size x max_len x num_tags，特征矩阵。
+        :param mask: ByteTensor, batch_size x max_len, 为0的位置认为是pad；如果为None，则认为没有padding。
+        :param unpad: bool, 是否将结果删去padding,
+                        False, 返回的是batch_size x max_len的tensor，
+                        True，返回的是List[List[int]], 内部的List[int]为每个sequence的label，已经除去pad部分，即每个List[int]
+                            的长度是这个sample的有效长度。
+        :return: 返回 (paths, scores)。
+                    paths: 是解码后的路径, 其值参照unpad参数.
+                    scores: torch.FloatTensor, size为(batch_size,), 对应每个最优路径的分数。
 
         """
-        batch_size, seq_len, n_tags = data.size()
-        data = data.transpose(0, 1).data # L, B, H
+        batch_size, seq_len, n_tags = feats.size()
+        feats = feats.transpose(0, 1).data # L, B, H
         mask = mask.transpose(0, 1).data.byte() # L, B
 
         # dp
-        vpath = data.new_zeros((seq_len, batch_size, n_tags), dtype=torch.long)
-        vscore = data[0]
+        vpath = feats.new_zeros((seq_len, batch_size, n_tags), dtype=torch.long)
+        vscore = feats[0]
         transitions = self._constrain.data.clone()
         transitions[:n_tags, :n_tags] += self.trans_m.data
         if self.include_start_end_trans:
@@ -285,23 +281,24 @@ class ConditionalRandomField(nn.Module):
         trans_score = transitions[:n_tags, :n_tags].view(1, n_tags, n_tags).data
         for i in range(1, seq_len):
             prev_score = vscore.view(batch_size, n_tags, 1)
-            cur_score = data[i].view(batch_size, 1, n_tags)
+            cur_score = feats[i].view(batch_size, 1, n_tags)
             score = prev_score + trans_score + cur_score
             best_score, best_dst = score.max(1)
             vpath[i] = best_dst
             vscore = best_score.masked_fill(mask[i].eq(0).view(batch_size, 1), 0) + \
                                 vscore.masked_fill(mask[i].view(batch_size, 1), 0)
 
-        vscore += transitions[:n_tags, n_tags+1].view(1, -1)
+        if self.include_start_end_trans:
+            vscore += transitions[:n_tags, n_tags+1].view(1, -1)
 
         # backtrace
-        batch_idx = torch.arange(batch_size, dtype=torch.long, device=data.device)
-        seq_idx = torch.arange(seq_len, dtype=torch.long, device=data.device)
+        batch_idx = torch.arange(batch_size, dtype=torch.long, device=feats.device)
+        seq_idx = torch.arange(seq_len, dtype=torch.long, device=feats.device)
         lens = (mask.long().sum(0) - 1)
         # idxes [L, B], batched idx from seq_len-1 to 0
         idxes = (lens.view(1,-1) - seq_idx.view(-1,1)) % seq_len
 
-        ans = data.new_empty((seq_len, batch_size), dtype=torch.long)
+        ans = feats.new_empty((seq_len, batch_size), dtype=torch.long)
         ans_score, last_tags = vscore.max(1)
         ans[idxes[0], batch_idx] = last_tags
         for i in range(seq_len - 1):
diff --git a/fastNLP/modules/decoder/utils.py b/fastNLP/modules/decoder/utils.py
new file mode 100644
index 00000000..6e35af9a
--- /dev/null
+++ b/fastNLP/modules/decoder/utils.py
@@ -0,0 +1,70 @@
+
+import torch
+
+
+def log_sum_exp(x, dim=-1):
+    max_value, _ = x.max(dim=dim, keepdim=True)
+    res = torch.log(torch.sum(torch.exp(x - max_value), dim=dim, keepdim=True)) + max_value
+    return res.squeeze(dim)
+
+
+def viterbi_decode(feats, transitions, mask=None, unpad=False):
+    """给定一个特征矩阵以及转移分数矩阵，计算出最佳的路径以及对应的分数
+
+    :param feats: FloatTensor, batch_size x max_len x num_tags，特征矩阵。
+    :param transitions: FloatTensor, n_tags x n_tags。[i, j]位置的值认为是从tag i到tag j的转换。
+    :param mask: ByteTensor, batch_size x max_len, 为0的位置认为是pad；如果为None，则认为没有padding。
+    :param unpad: bool, 是否将结果删去padding,
+                False, 返回的是batch_size x max_len的tensor，
+                True，返回的是List[List[int]], 内部的List[int]为每个sequence的label，已经除去pad部分，即每个List[int]的长度是
+                    这个sample的有效长度。
+    :return: 返回 (paths, scores)。
+                paths: 是解码后的路径, 其值参照unpad参数.
+                scores: torch.FloatTensor, size为(batch_size,), 对应每个最优路径的分数。
+
+    """
+    batch_size, seq_len, n_tags = feats.size()
+    assert n_tags==transitions.size(0) and n_tags==transitions.size(1), "The shapes of transitions and feats are not " \
+                                                                        "compatible."
+    feats = feats.transpose(0, 1).data  # L, B, H
+    if mask is not None:
+        mask = mask.transpose(0, 1).data.byte()  # L, B
+    else:
+        mask = feats.new_ones((seq_len, batch_size), dtype=torch.uint8)
+
+    # dp
+    vpath = feats.new_zeros((seq_len, batch_size, n_tags), dtype=torch.long)
+    vscore = feats[0]
+
+    vscore += transitions[n_tags, :n_tags]
+    trans_score = transitions[:n_tags, :n_tags].view(1, n_tags, n_tags).data
+    for i in range(1, seq_len):
+        prev_score = vscore.view(batch_size, n_tags, 1)
+        cur_score = feats[i].view(batch_size, 1, n_tags)
+        score = prev_score + trans_score + cur_score
+        best_score, best_dst = score.max(1)
+        vpath[i] = best_dst
+        vscore = best_score.masked_fill(mask[i].eq(0).view(batch_size, 1), 0) + \
+                 vscore.masked_fill(mask[i].view(batch_size, 1), 0)
+
+    # backtrace
+    batch_idx = torch.arange(batch_size, dtype=torch.long, device=feats.device)
+    seq_idx = torch.arange(seq_len, dtype=torch.long, device=feats.device)
+    lens = (mask.long().sum(0) - 1)
+    # idxes [L, B], batched idx from seq_len-1 to 0
+    idxes = (lens.view(1, -1) - seq_idx.view(-1, 1)) % seq_len
+
+    ans = feats.new_empty((seq_len, batch_size), dtype=torch.long)
+    ans_score, last_tags = vscore.max(1)
+    ans[idxes[0], batch_idx] = last_tags
+    for i in range(seq_len - 1):
+        last_tags = vpath[idxes[i], batch_idx, last_tags]
+        ans[idxes[i + 1], batch_idx] = last_tags
+    ans = ans.transpose(0, 1)
+    if unpad:
+        paths = []
+        for idx, seq_len in enumerate(lens):
+            paths.append(ans[idx, :seq_len + 1].tolist())
+    else:
+        paths = ans
+    return paths, ans_score
\ No newline at end of file
diff --git a/reproduction/Chinese_word_segmentation/models/cws_model.py b/reproduction/Chinese_word_segmentation/models/cws_model.py
index daefc380..13632207 100644
--- a/reproduction/Chinese_word_segmentation/models/cws_model.py
+++ b/reproduction/Chinese_word_segmentation/models/cws_model.py
@@ -183,7 +183,7 @@ class CWSBiLSTMCRF(BaseModel):
         masks = seq_lens_to_mask(seq_lens)
         feats = self.encoder_model(chars, bigrams, seq_lens)
         feats = self.decoder_model(feats)
-        probs = self.crf.viterbi_decode(feats, masks, get_score=False)
+        paths, _ = self.crf.viterbi_decode(feats, masks)
 
-        return {'pred': probs, 'seq_lens':seq_lens}
+        return {'pred': paths, 'seq_lens':seq_lens}
 
diff --git a/reproduction/Chinese_word_segmentation/models/cws_transformer.py b/reproduction/Chinese_word_segmentation/models/cws_transformer.py
index 736edade..d49ce3a9 100644
--- a/reproduction/Chinese_word_segmentation/models/cws_transformer.py
+++ b/reproduction/Chinese_word_segmentation/models/cws_transformer.py
@@ -72,9 +72,9 @@ class TransformerCWS(nn.Module):
         feats = self.transformer(x, masks)
         feats = self.fc2(feats)
 
-        probs = self.crf.viterbi_decode(feats, masks, get_score=False)
+        paths, _ = self.crf.viterbi_decode(feats, masks)
 
-        return {'pred': probs, 'seq_lens':seq_lens}
+        return {'pred': paths, 'seq_lens':seq_lens}
 
 
 class NoamOpt(torch.optim.Optimizer):