diff --git a/python/paddle/v2/fluid/backward.py b/python/paddle/v2/fluid/backward.py
index 416d2ae7853d150d898ceb0fa01f37416e431b2e..b12767b3bbbe51f2f60aede8470b9cff7e7d2c4c 100644
--- a/python/paddle/v2/fluid/backward.py
+++ b/python/paddle/v2/fluid/backward.py
@@ -71,7 +71,9 @@ def _backward_impl_(target,
         if each_op.has_attr("sub_block"):
             sub_block_idx = each_op.block_attr("sub_block")
             sub_block = program.block(sub_block_idx)
+            original_block_idx = program.current_block_idx
             grad_sub_block = program.create_block(parent_idx=sub_block_idx)
+            program.current_block_idx = original_block_idx
             _backward_impl_(target, sub_block, grad_sub_block, no_grad_set,
                             grad_info_map, callback)
             grad_sub_block_list.append(grad_sub_block.desc)
@@ -120,9 +122,9 @@ def _backward_impl_(target,
             pending_sum_ops.append((_create_op_desc_(
                 op_type="sum",
                 inputs={"X": inputs},
-                outputs={"Out": var_name},
+                outputs={"Out": [var_name]},
                 attrs={}), len(grad_op_descs)))
-    # 根据append的顺序可以看出pending_sum_ops一定是根据sum_op的插入位置排序的
+    # sum_op descs are sorted according to their insert position
     for p in reversed(pending_sum_ops):
         grad_op_descs.insert(p[1], p[0])
     # Remove ops whose outputs are all in no_grad_set