Using the two-way nested grid mesoscale model (MM5), a heavy rain event over the middle and the lower reaches of the Changjiang River on the last ten days of June in 1999 is simulated. It is shown that the MM5 model can successfully simulate the event. The slowly motion of the meso-β vortex is the basic cause for the event. The high-level atmosphere divergence and the low-level atmosphere convergence, the strong vertical motion, the low-level plenty vapor source and the releasing of the convective unstable energy are of the dynamical mechnism of the vortex and of the maintaining and growing of the rain.