Heavy precipitation event during 4—5 July 2003 is diagnosed by using the observational data.Based on mesoscale model MM5(V3.6),the meso-β-scale system is simulated.The simulation shows that the foundamental reason for the cease of the precipitation is not change in ascending motion condition,but the gradual replacement of wet air by dry air in the low layer in the rainstorm area under influence of the mid-low level horizontal circulation and the secondary circulation of the Meiyu front.