diff --git a/ppdet/utils/coco_eval.py b/ppdet/utils/coco_eval.py
index fa86c292a62050d8c4530e9b29e7a1aa4fe088ce..d2ceeacb42e6575d7e7dbbeeb2b4a59f86493634 100644
--- a/ppdet/utils/coco_eval.py
+++ b/ppdet/utils/coco_eval.py
@@ -121,6 +121,7 @@ def mask_eval(results, anno_file, outfile, resolution, thresh_binarize=0.5):
             im_id = int(im_ids[i][0])
             clsid_scores = bboxes[s:s + num][:, 0:2]
             mask = masks[s:s + num]
+            s += num
             for j in range(num):
                 clsid, score = clsid_scores[j].tolist()
                 catid = int(clsid2catid[clsid])