A rainstorm occurred in Yangtze-Huaihe River basin on July 5, 2018, was simulated with the mesoscale model of Weather Research and Forecast (WRF). Results show that the location of the precipitation area, intensity, and large value centers successfully, which could describe the occurrence and development of the rainstorm and mesoscale system. The fine grid data output from the model was used for diagnostic analysis. Results indicate that the coupling of high and low air jets, the convergence of lower layer, the divergence of upper layer, and the strong ascending motion to the tropopause caused by the whole positive vorticity tube provide favorable dynamic conditions for the occurrence and development of heavy precipitation and squall line system in rainfall area. The significant wet region, the southwest moisture transport with convergence, the potential instability along with the high energy region provide the vapor and thermal conditions. The analysis of the moist potential vorticity shows the particularity of this rainstorm. One is the unstable atmosphere is mainly concentrated at low altitude. In addition, both barotropic part (MPV1) and baroclinic part of moist potential vorticity (MPV2) were stuck in the middle of low value regions on the east and west sides, which was the reason why the rainstorm developed violently and maintained a short time.