Adopting a high resolution nested grid scheme and proper physical parameters,a Mei-yu heavy rain process in the middle reach of the Yangtze River from 0800 BST July 22 to 0800 BST July 23,2002 is simulated by use of PSU/NCAR mesoscale nonstatic numerical forecast model MM5 in this paper.The simulation results successfully describe the spatial and temporal distributions of the rain process and the developing course of the concomitant meso-β-scale system.The result shows that a series of meso-β-scale systems developing along the Mei-yu shear line are the direct influencing systems of this heavy rain process.The existence and changing fashion of the mesoscale divergence systems at high levels affect greatly the development of the meso-β-scale systems.The outgrowth of the mesoscale vortexes at low and middle levels could lead to the appearance of smaller scale strong cold and warm advection,which excites the baroclinic development of the mesoscale systems themselves.What's more,the latent heat of precipitation in turn have an important feedback effect to the development and maintenance of the heavy rain process,too.