Developed on the basis of the limited-area fine mesh model of Zhou Xiaoping is a nested-grid model by which numerical simulation and dynamic analysis are performed of an exception-ally heavy rain event over the Channgjiang-Huaihe basins during July 2～3, 1991. Results showthat low-level jet is enhanced, transporting large quantities of warm, moist air into the system,which by exciting intense rising, is responsible mainly for the rainstorm, during which thermalwind equilibrium is destroyed and a secondary circulation related to the equilibrium portion of thenonthermal wind favors the occurrence of vertical motion.