A strong rainstorm occurred at Shengsi in the north of Zhoushan city,Zhejiang Province from 20:00 August 5th to 20:00 August 6th 2001. Meantime, a torrential rainstorm happened in Shanghai as well,its rainfall was up to 275mm. Using routine observation data, the large scale atmospheric circulation and corresponding physical fields associated with this rain event are analyzed. The results show that the subtropical high is anomalously strong in earlier time. The easterly wind in the south side of subtropical high provides a plenty of water vapor and energy. In the north, the westerly trough carrying weak cold air intrudes into the remains of a tropical depression, which generates strong convection and triggers the release of unstable energy. It leads to the development of mesoscale and smallscale systems and provides the dynamic lifting, which is necessary for rainstorm to develop.