By using TBB data retrieved from FY2E satellite, Doppler radar data, mesoscale automatic observation data and fully assimilated model data with 5 km resolution from WRF model, a shorttime severe rainstorm in the north of Hangzhou Bay on 21 July 2013 is analyzed. The results indicate that the occurrence of the convective storm is related to the convergence of westsouthwest and eastsoutheast flows near the surface. The westerly outflow caused by local thermal convection in the east of Huzhou enchances the original intensity of the westerly flow in the west of Jiaxing, which is a very important factor to trigger the convection. When the convection develops to some extent, the downward current emerges in the lower troposphere. Part of it flows into east current near the surface and induces enchanced convergence in the lower troposphere and new updraft in the midtroposphere. Because of the weak vertical shear of horizontal wind from surface to 3 km above ground surface prior to the convection, the lifetime of cell thunderstorm in this event is short. In the vicinity of rainstorm the state of atmosphere near surface is neural or stable and the equivalent potential temperature is decreasing significantly above the lower atmosphere.