diff --git a/paddle/fluid/operators/detection/box_coder_op.cc b/paddle/fluid/operators/detection/box_coder_op.cc
index 5db600b19a314094df333908ef37a8c5619e4a50..e3424174915b20aa7a1443b6f3781d99858d5f15 100644
--- a/paddle/fluid/operators/detection/box_coder_op.cc
+++ b/paddle/fluid/operators/detection/box_coder_op.cc
@@ -166,7 +166,11 @@ where `tx`, `ty`, `tw`, `th` denote the target box's center coordinates, width
 and height respectively. Similarly, `px`, `py`, `pw`, `ph` denote the
 priorbox's (anchor) center coordinates, width and height. `pxv`, `pyv`, `pwv`,
 `phv` denote the variance of the priorbox and `ox`, `oy`, `ow`, `oh` denote the
-encoded/decoded coordinates, width and height.
+encoded/decoded coordinates, width and height. 
+
+During Box Decoding, two modes for broadcast are supported. Say target box has 
+shape [N, M, 4], and the shape of prior box can be [N, 4] or [M, 4]. Then prior
+box will broadcast to target box along the assigned axis. 
 )DOC");
   }
 };