Based on the WRF(weather research and forecasting) model output data with high spatial and temporal resolutions, the NCEP/NCAR 1°×1° reanalysis data, and the ground station precipitation data, this paper analyses a heavy rainstorm event in Sichuan Province from 31 July to 1 August 2009.Results show that the rainstorm bands approximately locate along the water vapor convergence regions, and the divergence field, water vapor flux and its divergence have excellent indicative function in the rainfall event.However, the atmospheric stratification is weakly stable for six hours before precipitation occurrence, which is not conductive to the occurrence and development of convection.But the analysis of the divergence field, the nonlinear balance equation and Richardson number shows that the mesoscale gravity waves exist before the rain begins, which induce the development of convective motion, while the convection plays a major role in stimulating the development of the rainstorm.Therefore, during the process, we must consider the interaction between the mesoscale gravity waves and convective motion, in order to explain the reason why the rain bands develop and maintain reasonably.