diff --git a/mmdet/datasets/pipelines/transforms.py b/mmdet/datasets/pipelines/transforms.py
index 60ee42de69cd85845ea9ac8731a8d12bda7db715..621be4d4afe2332b3392ea61f989637e6c18634e 100644
--- a/mmdet/datasets/pipelines/transforms.py
+++ b/mmdet/datasets/pipelines/transforms.py
@@ -527,7 +527,7 @@ class Expand(object):
         left = int(random.uniform(0, w * ratio - w))
         top = int(random.uniform(0, h * ratio - h))
         expand_img[top:top + h, left:left + w] = img
-        boxes += np.tile((left, top), 2)
+        boxes = boxes + np.tile((left, top), 2)
 
         results['img'] = expand_img
         results['gt_bboxes'] = boxes