A heavy rainstorm occurring in the southern part of Jiangsu Province including ShanghaiNanjing Expressway on September 17th,2009 is simulated by using WRF22(Weather Research and Forecasting Model for version 22) and NCEP(National Center of Environment Predicting) reanalyzed meteorological data with 1°×1° grid.The verification of the observed data from the Automatic Weather Monitoring System on the ShanghaiNanjing Expressway displays that the results of the simulation on the rainstorm process are satisfactory and the simulations coincide with the observed values.Diagnoses are made based on the output data from the model.The results are shown as follows:(1)The development and movement of a meso〖WTBZ〗β〖WTB1〗scale low vortex in the middle and lower reaches of the Yangtze River play an important role in the enhancement and maintenance of the rainfall in the process;(2)The moisture convergence zone below 500 hPa is significant and it provides important vapor conditions for the unstable growth of convection and the maintaining of rainstorm by the forming of the deep and humid environment in the rainfall area;(3)There is strong vertical motion in the rainfall area and the vertical structure configuration of the vorticity field is consistent with that of the divergence field.As a result,the unstable energy in the atmosphere release and the strong convective weather forms;(4)The atmospheric stratification is neutral in the middle and upper of troposphere but potential instable in the lower atmosphere.So there is convection development in the whole layer of the atmosphere,which is conducive to the formation of the rainstorm.