diff --git a/fastNLP/core/losses.py b/fastNLP/core/losses.py
index 1e5a4914..39ba4012 100644
--- a/fastNLP/core/losses.py
+++ b/fastNLP/core/losses.py
@@ -1,20 +1,76 @@
 import torch
 
+from fastNLP.core.utils import _get_arg_list
+from fastNLP.core.utils import _map_args
+from fastNLP.core.utils import get_func_signature
+from fastNLP.core.utils import _build_args
+
 
 class LossBase(object):
     def __init__(self):
+        # key: name in target function; value: name in output function
         self.param_map = {}
 
     def get_loss(self, *args, **kwargs):
         raise NotImplementedError
 
-    def __call__(self, output_dict, predict_dict):
-        pass
+    def __call__(self, output_dict, target_dict):
+        """
+        :param output_dict: A dict from forward function of the network.
+        :param target_dict: A dict from DataSet.batch_y.
+        :return:
+        """
+        args, defaults, defaults_val, varargs, kwargs = _get_arg_list(self.get_loss)
+        if varargs is not None:
+            raise RuntimeError(
+                f"The function {get_func_signature(self.get_loss)} should not use Positional Argument."
+            )
+
+        param_map = self.param_map
+        for keys in args:
+            if keys not in param_map:
+                param_map.update({keys: keys})
+        for keys in defaults:
+            if keys not in param_map:
+                param_map.update({keys: keys})
+        # param map: key= name in get_loss function, value= name in param dict
+        reversed_param_map = {val: key for key, val in param_map}
+        # reversed param map: key= name in param dict, value= name in get_loss function
+
+        param_val_dict = {}
+        for keys, val in output_dict.items():
+            if keys not in target_dict.keys():
+                param_val_dict.update({keys: val})
+            else:
+                raise RuntimeError("conflict Error in output dict and target dict with name {}".format(keys))
+        for keys, val in target_dict.items():
+            if keys not in output_dict.keys():
+                param_val_dict.update({keys: val})
+            else:
+                raise RuntimeError("conflict Error in output dict and target dict with name {}".format(keys))
 
+        for keys in args:
+            if param_map[keys] not in param_val_dict.keys():
+                raise RuntimeError("missing param {} in function {}".format(keys, self.get_loss))
 
-class Loss(LossBase):
-    def __init__(self):
-        pass
+        param_map_val = _map_args(reversed_param_map, **param_val_dict)
+        param_value = _build_args(**param_map_val)
+
+        loss = self.get_loss(**param_value)
+
+        if not (isinstance(loss, torch.Tensor) and len(loss.size()) == 0):
+            if not isinstance(loss, torch.Tensor):
+                raise RuntimeError("loss ERROR: loss except a torch.Tensor but get {}".format(type(loss)))
+            raise RuntimeError("loss ERROR: len(loss.size()) except 0 but got {}".format(len(loss.size())))
+
+        return loss
+
+
+class NewLoss(LossBase):
+    def __init__(self, func, key_map=None, **kwargs):
+        super(NewLoss).__init__()
+        if not callable(func):
+            raise RuntimeError("")
 
 
 def squash(predict, truth, **kwargs):
diff --git a/fastNLP/core/utils.py b/fastNLP/core/utils.py
index 84faaece..13982e27 100644
--- a/fastNLP/core/utils.py
+++ b/fastNLP/core/utils.py
@@ -64,6 +64,39 @@ def _build_args(func, **kwargs):
     return output
 
 
+def _map_args(maps: dict, **kwargs):
+    # maps: key=old name, value= new name
+    output = {}
+    for name, val in kwargs.items():
+        if name in maps:
+            assert isinstance(maps[name], str)
+            output.update({maps[name]: val})
+        else:
+            output.update({name: val})
+    for keys in maps.keys():
+        if keys not in output.keys():
+            # TODO: add UNUSED warning.
+            pass
+    return output
+
+
+def _get_arg_list(func):
+    assert callable(func)
+    spect = inspect.getfullargspec(func)
+    if spect.defaults is not None:
+        args = spect.args[: -len(spect.defaults)]
+        defaults = spect.args[-len(spect.defaults):]
+        defaults_val = spect.defaults
+    else:
+        args = spect.args
+        defaults = None
+        defaults_val = None
+    varargs = spect.varargs
+    kwargs = spect.varkw
+    return args, defaults, defaults_val, varargs, kwargs
+
+
+
 # check args
 def _check_arg_dict_list(func, args):
     if isinstance(args, dict):