diff --git a/python/paddle_serving_server/pyserver.py b/python/paddle_serving_server/pyserver.py
index e9bc43be7d6f7131526e3f38ff9422f1c2c42143..1a776ecaefd528cb6f3161c215c5b5cae1e7cb07 100644
--- a/python/paddle_serving_server/pyserver.py
+++ b/python/paddle_serving_server/pyserver.py
@@ -1004,6 +1004,7 @@ class PyServer(object):
                 op.name = "#G"  # update read_op.name
                 break
         outdegs = {op.name: [] for op in self._user_ops}
+        zero_indeg_num, zero_outdeg_num = 0, 0
         for idx, op in enumerate(self._user_ops):
             # check the name of op is globally unique
             if op.name in indeg_num:
@@ -1011,8 +1012,16 @@ class PyServer(object):
             indeg_num[op.name] = len(op.get_input_ops())
             if indeg_num[op.name] == 0:
                 ques[que_idx].put(op)
+                zero_indeg_num += 1
             for pred_op in op.get_input_ops():
                 outdegs[pred_op.name].append(op)
+        if zero_indeg_num != 1:
+            raise Exception("DAG contains multiple input Ops")
+        for _, succ_list in outdegs.items():
+            if len(succ_list) == 0:
+                zero_outdeg_num += 1
+        if zero_outdeg_num != 1:
+            raise Exception("DAG contains multiple output Ops")
 
         # topo sort to get dag_views
         dag_views = []
@@ -1035,10 +1044,6 @@ class PyServer(object):
             que_idx = (que_idx + 1) % 2
         if sorted_op_num < len(self._user_ops):
             raise Exception("not legal DAG")
-        if len(dag_views[0]) != 1:
-            raise Exception("DAG contains multiple input Ops")
-        if len(dag_views[-1]) != 1:
-            raise Exception("DAG contains multiple output Ops")
 
         # create channels and virtual ops
         def name_generator(prefix):
@@ -1121,7 +1126,7 @@ class PyServer(object):
                         processed_op.add(other_op.name)
         output_channel = Channel(self._manager, name=channel_name_gen.next())
         channels.append(output_channel)
-        last_op = dag_views[-1][0]  # TODO: fix it
+        last_op = dag_views[-1][0]
         last_op.add_output_channel(output_channel)
 
         self._actual_ops = virtual_ops
diff --git a/python/paddle_serving_server_gpu/pyserver.py b/python/paddle_serving_server_gpu/pyserver.py
index e9bc43be7d6f7131526e3f38ff9422f1c2c42143..1a776ecaefd528cb6f3161c215c5b5cae1e7cb07 100644
--- a/python/paddle_serving_server_gpu/pyserver.py
+++ b/python/paddle_serving_server_gpu/pyserver.py
@@ -1004,6 +1004,7 @@ class PyServer(object):
                 op.name = "#G"  # update read_op.name
                 break
         outdegs = {op.name: [] for op in self._user_ops}
+        zero_indeg_num, zero_outdeg_num = 0, 0
         for idx, op in enumerate(self._user_ops):
             # check the name of op is globally unique
             if op.name in indeg_num:
@@ -1011,8 +1012,16 @@ class PyServer(object):
             indeg_num[op.name] = len(op.get_input_ops())
             if indeg_num[op.name] == 0:
                 ques[que_idx].put(op)
+                zero_indeg_num += 1
             for pred_op in op.get_input_ops():
                 outdegs[pred_op.name].append(op)
+        if zero_indeg_num != 1:
+            raise Exception("DAG contains multiple input Ops")
+        for _, succ_list in outdegs.items():
+            if len(succ_list) == 0:
+                zero_outdeg_num += 1
+        if zero_outdeg_num != 1:
+            raise Exception("DAG contains multiple output Ops")
 
         # topo sort to get dag_views
         dag_views = []
@@ -1035,10 +1044,6 @@ class PyServer(object):
             que_idx = (que_idx + 1) % 2
         if sorted_op_num < len(self._user_ops):
             raise Exception("not legal DAG")
-        if len(dag_views[0]) != 1:
-            raise Exception("DAG contains multiple input Ops")
-        if len(dag_views[-1]) != 1:
-            raise Exception("DAG contains multiple output Ops")
 
         # create channels and virtual ops
         def name_generator(prefix):
@@ -1121,7 +1126,7 @@ class PyServer(object):
                         processed_op.add(other_op.name)
         output_channel = Channel(self._manager, name=channel_name_gen.next())
         channels.append(output_channel)
-        last_op = dag_views[-1][0]  # TODO: fix it
+        last_op = dag_views[-1][0]
         last_op.add_output_channel(output_channel)
 
         self._actual_ops = virtual_ops