With the meso-scale non-hydrostatic model MM5,a heavy rainstorm in the Jiangnan area in the Meiyu peroid of 2003 has been simulated and the formation mechanism of the rainstorm analyzed.Result shows that the rainstorm belonged to a shearline-causing rainstorm.There were an energy front zone in the low level,and some inertial instable areas in the upper level of troposphere during the torrential heavy rain,and heavy rain took place on the left front side of a low level jet stream,and the right front side of an upper level jet stream.By analyzing the changes of physical quantities with time and height at the rainfall center,it can be seen that the cyclonic vorticity and convergence at the low level,the anticyclonic vorticity and divergence at the upper level,deep upward motion,the lifting of the isolines of equivalent temperature,the magnitude of negative ξ_mpv,ξ_mpv1 and ξ_mpv2 becoming larger were all favorable conditions to the heavy rain.When the rain weakened,almost all these physical quantities went to the opposite.The direct influencing weather systems showed the spatial and temporal features on meso-scale.It was clear that during the heavy rain there existed obvious inertial instability in the upper levels and convective instability in the lower levels.