The Meiyu front rainstorm occurred from July 4 to July 5,2003 in the Yangtze-Huaihe River basin was simulated and diagnosed by WRF.The analysis results suggested that the torrential rain region was in an environment of high temperature and high humidity.The rainstorm process was promoted by the coupling of upper and lower tropospheric jet stream,and the combination of convergence in lower troposphere and divergence in upper troposphere.The moist potential vorticity was computed to reveal the physical mechanisms of this rainstorm.The overlapping of the positive and negative ζ_MPV1 in upper and lower level,and the evolution of ζ_MPV1<0 and ζ_MPV2>0 showed that unstable convective energy as well as slantwise vorticity developed in this rainstorm process.ζ_MPV1 and ζ_MPV2 totally suggest the intensification of convective instability and baroclinic instability in the storm area.