Based on the analysis of the potential vorticity(PV) on isentropic su rface in the three phases of regional rainstorm in Henan Province caused by Typh oon Haitang, the mechanism for the generation of the rainstorm is disclosed. The results show that typhoon is a system with high PV value, the first rainstorm p hase is generated in the south to the southeasterly jet stream which lies in the northeastern part of typhoon. The second and the third phases are the rainstorm s far from the typhoon. They occur because of the interaction of typhoon and col d air with high PV when typhoon reaches the relatively high latitude. In additio n, the southward shift of upper level jet stream is advantageous to the southwar d transportation of high value PV and can promote the intensification of rainsto rm.