diff --git a/examples/bmn/bmn_metric.py b/examples/bmn/bmn_metric.py
index f9bf101f825913572803fbb1168260f83a0d96ac..d8e0d3e3ae307c9fa61000e336b4ef6864f956f2 100644
--- a/examples/bmn/bmn_metric.py
+++ b/examples/bmn/bmn_metric.py
@@ -39,11 +39,6 @@ class BmnMetric(Metric):
         elif self.mode == 'infer':
             self.get_infer_dataset_dict()
 
-    def add_metric_op(self, preds, label):
-        pred_bm, pred_start, pred_en = preds
-        video_index = label[-1]
-        return [pred_bm, pred_start, pred_en, video_index]  #return list
-
     def update(self, pred_bm, pred_start, pred_end, fid):
         # generate proposals
         pred_start = pred_start[0]
diff --git a/hapi/metrics.py b/hapi/metrics.py
index 3350853677b62275bb0107addff3f3b3780ea81c..1d24c4ada2e77bba0df59cad75dd0fac3842f80c 100644
--- a/hapi/metrics.py
+++ b/hapi/metrics.py
@@ -48,9 +48,16 @@ class Metric(object):
                                   format(self.__class__.__name__))
 
     @abc.abstractmethod
-    def update(self, *args, **kwargs):
+    def update(self, *args):
         """
         Update states for metric
+
+        Inputs of :code:`update` is the outputs of :code:`Metric.add_metric_op`,
+        if :code:`add_metric_op` is not defined, the inputs of :code:`update`
+        will be flatten arguments of **output** of mode and **label** from data:
+        :code:`update(output1, output2, ..., label1, label2,...)`
+
+        see :code:`Metric.add_metric_op`
         """
         raise NotImplementedError("function 'update' not implemented in {}.".
                                   format(self.__class__.__name__))
@@ -72,11 +79,26 @@ class Metric(object):
         raise NotImplementedError("function 'name' not implemented in {}.".
                                   format(self.__class__.__name__))
 
-    def add_metric_op(self, pred, label):
+    def add_metric_op(self, *args):
         """
-        Add process op for metric in program
+        This API is advanced usage to accelerate metric calculating, calulations
+        from outputs of model to the states which should be updated by Metric can
+        be defined here, where Paddle OPs is also supported. Outputs of this API
+        will be the inputs of "Metric.update".
+
+        If :code:`add_metric_op` is defined, it will be called with **outputs**
+        of model and **labels** from data as arguments, all outputs and labels
+        will be concatenated and flatten and each filed as a separate argument
+        as follows:
+        :code:`add_metric_op(output1, output2, ..., label1, label2,...)`
+
+        If :code:`add_metric_op` is not defined, default behaviour is to pass
+        input to output, so output format will be:
+        :code:`return output1, output2, ..., label1, label2,...`
+
+        see :code:`Metric.update`
         """
-        return pred, label
+        return args
 
 
 class Accuracy(Metric):
@@ -91,12 +113,12 @@ class Accuracy(Metric):
         self._init_name(name)
         self.reset()
 
-    def add_metric_op(self, pred, label, *args, **kwargs):
-        pred = fluid.layers.argsort(pred[0], descending=True)[1][:, :self.maxk]
-        correct = pred == label[0]
+    def add_metric_op(self, pred, label, *args):
+        pred = fluid.layers.argsort(pred, descending=True)[1][:, :self.maxk]
+        correct = pred == label
         return correct
 
-    def update(self, correct, *args, **kwargs):
+    def update(self, correct, *args):
         accs = []
         for i, k in enumerate(self.topk):
             num_corrects = correct[:, :k].sum()
diff --git a/hapi/model.py b/hapi/model.py
index e2fa2d6e3ba900e783421d0e8b29fa9a3aad5813..f4e6744df5107d345c873f6fa45269f704615708 100644
--- a/hapi/model.py
+++ b/hapi/model.py
@@ -67,7 +67,7 @@ def to_list(value):
     if value is None:
         return value
     if isinstance(value, (list, tuple)):
-        return value
+        return list(value)
     return [value]
 
 
@@ -473,7 +473,7 @@ class StaticGraphAdapter(object):
             if mode != 'test':
                 for metric in self.model._metrics:
                     metrics.append(
-                        to_list(metric.add_metric_op(outputs, labels)))
+                        to_list(metric.add_metric_op(*(outputs + labels))))
 
             if mode == 'train' and self.model._optimizer:
                 self._loss_endpoint = fluid.layers.sum(losses)
@@ -593,7 +593,7 @@ class DynamicGraphAdapter(object):
         metrics = []
         for metric in self.model._metrics:
             metric_outs = metric.add_metric_op(
-                to_list(outputs), to_list(labels))
+                *(to_list(outputs) + to_list(labels)))
             m = metric.update(*[to_numpy(m) for m in to_list(metric_outs)])
             metrics.append(m)
 
@@ -632,7 +632,8 @@ class DynamicGraphAdapter(object):
                     self._merge_count[self.mode + '_total'] += samples
                     self._merge_count[self.mode + '_batch'] = samples
 
-            metric_outs = metric.add_metric_op(to_list(outputs), labels)
+            metric_outs = metric.add_metric_op(
+                *(to_list(outputs) + to_list(labels)))
             m = metric.update(*[to_numpy(m) for m in to_list(metric_outs)])
             metrics.append(m)