The heavy rain occurred from 2000BST July 4 to 2000BST July 5,2003 in the Yangtze-Huaihe River Basin was simulated by the mesoscale numerical model MM5.The analysis suggests that the heavy rain event was close related to the generation and development of a mesoscale vortex in the mid-low troposphere.The vortex moved together with a mesoscale rain cluster,and its intensity was in agreement with the evolution of precipitation intesity at the center of the rain cluster.The disposition of the intensive ascending motion at the center of the vortex,the lower-level convergence and upper-level divergence,and the accumulation of unstable energy in the low level of the vortex were very favorable for the generation and development of the mesoscale vortex.It was found that the higher helicity in the mid-low troposphere might be one of the possible mechanisms for the generation and development of the mesoscale vortex.