Based on the conventional ground observational data, a numerical simulation and moist potential vorticity (MPV) analysis has been carried on heavy rainfall event over Jiangxi province from 19 June to 20 June 2010, with a meso-scale rainstorm model. The results show that this rare rainstorm is a typical heavy rainfall over Meiyu front. The cold air flow behind North China vortex joined up the southwestern flow located in the northwest part of the strong and stable subtropical high, thus the cold air and warm air converged and maintained over the northern part of Hunan and Jiangxi provinces. The simulated precipitation of the high resolution model is very similar with the observational rainfall. The model has a good predictive skill for the location, intensity and center of heavy rainfall. By moist potential vorticity analysis, it is found that the distribution characteristic of MPV which heavy rainfall happens ahead has an obvious indication for precipitation forecast. The vertical overlapping of the positive and negative MPV1 areas is favorable to the generation and development of rainstorm. This zone is also the conjoint area of convective instability and baroclinic instability.