A Mei-Yu heavy rain event in the middle reach of the Changjiang River during July 22—23, 2002 is simulated by use of PSU/NCAR mesoscale nonstatic numerical forecast model MM5. The simulation result describes successfully the spatial and temporal rainfall distribution of the event and the developing course of the concomitant meso-β-scale systems. The analysis shows the characters of the high and low level stream field in various stage of the life cycle of the meso-β-sclale system.