Abstract: By using the NCEP reanalysis data,general observation data and the high resolution output data of MM5 model,numerical simulation and diagnoses have been performed for a rainstorm which occurred from 29 July to 31 July in 2009 in Sichuan basin.The results show that the rainstorm formed during the process of plateau vortex grew weaken as southwest vortex gathered strength.The water vapor of the rainstorm which flew into Sichuan basin through Guangdong and Guangxi mainly came from the South China Sea.Rainstorm center corresponded with positive center of MPV on isentropic surface.Pressure field and divergence field of moist isentropic surface sank in some degree and converged in the rainstorm area.MPV analysis on the isobaric surface shows that MPV1 positive area sank on the top level,the MPV1 changing from negative to positive in the middle and lower level of troposphere in the rainstorm area,and the MPV2 value was opposite to the MPV1 which changed from positive to negative in the middle and lower level of troposphere.Slantwise updraft and conditional symmetric instability were probably the trigger mechanism of the rainstorm.