The features of low latitude circulation and its role during the 2003 heavy rain period over the Huaihe River valley are analyzed. The results show that convergence of extremely strong southern wind and extremely strong northern wind caused the heavy rain. The strong southern wind composed mainly of south-eastern wind which transported warm moisture from tropical north-west Pacific to the Huaihe River valley. During the heavy rain period, the southwest monsoon is weaker than normal. The reason of the abnormal characteristics of weak southwest monsoon and strong southeast wind are also analyzed here.