Based on the weather pattern,combined with GMS 5 cloud image and WSR 88D radar data,primary analysis is given on the torrential rainstorm of Shanghai City during the night of August 5th 2001.It is found that MCS′ continuously updating and developing is the direct cause of severe rainstorm;helpful circulation leads to MCS′ formation and stabilization,special geographical condition of Shanghai,faced with water in three sides,provides MCS with plentiful water vapor.