Abstract: The mesoscale model MM4 was used to simulate the torrential rain associated with Meiyu front occurring in 5-6 July.1991 in the Changjiang-Huaihe Basin. Based on the outputs of the model. the cause of the mesoscale cyclogenesis on the lower troposphere has been investigated in terms of the potential vorticity principle. The results show that the favorable pattern of the moist isentropic surface and the cold air slided down along the moist isentropic surface rapidly were the favorable factors to increase the absolute vorticity, and to cause the cyclogenesis.