In this paper we analysed the circulation background, vapor transport and mesoscale convective system of an excessively heavy rain during 7-8 September 2010 at Suqian. The result showed that vapor convergence in Huaibei of Jiangsu due to the shear of east wind and southwest wind was beneficial to the heavy rain. The coupling of lower and upper jet, unstable stratification and cold air intrusion all were able to intensify the upward movement and vertical transportation of the water vapor. Mesoscale vortex, combined with low and airflow convergence, can be used to forecast where and when heavy rain may possibly happen. Also, they have great predictive significance for intensity and duration of heavy rain.