This paper carries out a numerical experiment on a flash-flood-producing rainstorm process from 3 to 5 September 2004 in Sichuan province,using NCEP/NCAR data and regional observational data as the background field.Because of the good precipitation simulation,the model outputs are used to analyze the non-linear convective symmetric instability of the storm.The results show that joint action of conditional instability and topographic forcing on the near-surface layer triggered the convection,then conditional symmetric instability at middle and high levels further promotes the convection.This process produced the vigorously developed deep moist convection and resulted in such a heavy and long-lasting rainstorm.