Abstract: Using MM4 model, the process of rainstorm is molded in the period from 08:00 BLT July 21 to 08:00 BLT July 22, 1998. It is found that the convergence and divergence field of the South Asia High and the northwesterly upper-level jet form a couple of mesoscale direct circulation and counter-circulation. The circulation pattern keeps the rainfall maintaining and strengthening. The sinking of the high potential vorticity is related to the lower vortex and the rainfall increasing. The conditional convective instability in the later period is more obvious for the effect of rainstorm than conditional symmetric instability.