diff --git a/PaddleCV/PaddleDetection/ppdet/modeling/roi_heads/cascade_head.py b/PaddleCV/PaddleDetection/ppdet/modeling/roi_heads/cascade_head.py
index aad4b141d6043ff0ca4dc66eaac5c58fd4986053..58936b3d4a07ab018d7b7268f92970d4e436ed35 100644
--- a/PaddleCV/PaddleDetection/ppdet/modeling/roi_heads/cascade_head.py
+++ b/PaddleCV/PaddleDetection/ppdet/modeling/roi_heads/cascade_head.py
@@ -197,7 +197,7 @@ class CascadeBBoxHead(object):
             # only use fg box delta to decode box
             bbox_pred_new = fluid.layers.slice(
                 bbox_pred_new, axes=[1], starts=[1], ends=[2])
-            bbox_pred_new = fluid.layers.expand(bbox_pred_new, [1, 81, 1])
+            bbox_pred_new = fluid.layers.expand(bbox_pred_new, [1, self.num_classes, 1])
         decoded_box = fluid.layers.box_coder(
             prior_box=proposals_boxes,
             prior_box_var=bbox_reg_w,