Using the intensive automatic weather station data, Doppler radar data, NCEP reanalysis data and the simulated results with mesoscale numerical model, the rainstorm occurred in Jiangsu and Anhui Provinces is studied. The precipitation evolution characteristics and formation mechanism of the rainstorm are also analyzed in detail. The results show that the boundary layer jet accompanying with rainstorm all the time accelerats development and enhancement of precipitation, especially it plays the important role indynamical and vapor transport condition of the formation of heavy rainstorm. The boundary layer jet disturbances is closelyconnected with the evolution of precipitation, the invasion of cold air from mid\| level apparently weaken the boundary layer jet, but intensified the atmospheric convection instability structure and vertical wind shear in rainstorm region, thus convective precipitation enhanced. After brief weakened, southerly in the rear of the boundary jet strengthen, which promote development of the shear line on its north side. During to the joint effects of them, local convergence and cyclonic vorticity greatly intensifyin rainstorm region, meanwhile a narrow and deep updraft is forced, which promote the development of mesoscale and microscale convective system in rainstorm region and vicinity and lead to a climax of precipitation. Besides, the boundary layer jet always goes with a water vapor flux convergence band and it is main water vapor transport channel. When the boundary layer jet develops, the water vapor flux convergence in rainstorm region enhances and precipitation developes as a result.