The paper diagnoses the wet bulb potential vorticity field,and analyzes the structural characters of the frontogenesis secondary circulation of a highly intensive precipitation event in 21th—25th July 2002 using the Sawyer-Eliassen secondary equation.Then based on above results,this paper discusses the formation and development mechanism of the event.The analysis shows that conditional symmetric instability might be the unstable mechanism responsible for the heavy rain event.A great deal of moisture carried by the strong southwest low jet was converged in the low layer of rain region,under the geostrophic momentum approximation,the frontogenetical secondary circulation driven by frontogenesis forcing made the warm and wet air ascend and the accumulated unstable energy released,thus leading to the occurrence of the intense precipitation.