diff --git a/ppdet/modeling/anchor_heads/retina_head.py b/ppdet/modeling/anchor_heads/retina_head.py
index cb6cb1cfbc22c7bd7957961ce609fd80e6efd4d1..eed81eb4de2e2fdb480b5e73f06c88c433ac8e3d 100644
--- a/ppdet/modeling/anchor_heads/retina_head.py
+++ b/ppdet/modeling/anchor_heads/retina_head.py
@@ -404,5 +404,5 @@ class RetinaHead(object):
             inside_weight=bbox_weight,
             outside_weight=bbox_weight)
         loss_bbox = fluid.layers.reduce_sum(loss_bbox, name='loss_bbox')
-        loss_bbox = loss_bbox / fg_num
+        loss_bbox = loss_bbox / fluid.layers.cast(fg_num, loss_bbox.dtype)
         return {'loss_cls': loss_cls, 'loss_bbox': loss_bbox}