A heavy rain event in Guizhou on June 18:00—19:00(UTC), 2002, is simulated with meso-scale numerical model(MM5). With the basic physical parameters from the model output, the potential equivalent temperature and vertical velocity are analyzed, and calculation of the helicity and the barotropic and baroclinic moist potential vorticity(MPV) are made. The diagnosis results are as follows. The center of the rainfall is adjacent to the maximum vertical velocity. The southern and northern meridional-vertical circulations are significant to the in- and out-flow on the lower and upper level, respectively. The change of the helicity is a good indicator for the evolution of the rainfall intensity. The heavy rainfall area lie in the dense part of helicity isograms. The occurrence, intensification and decay of the event is corresponding to that of the funnel-like MPV1 high value area. Moreover, the overlapping of the positive MPV1 at the upper-level and negative MPV1 at the low-level is advantageous to the appearance and development of the event. The positive MPV2 area at 500hPa is well corresponding to the convective rainfall area, and they have in phase changes.