Based on the routine sounding and surface observation data, NCEP grids data, MM5 model output, a diagnosis of the moist potential vorticity of the heavy rain event in the southern of North China on 4—5 July, 2000 is made. The results show that the spatial and temporal variations of moist potential vorticity can give good indication of the development of mesoscale vortex and heavy rain. It is advantage to the development of the vortex rainstorm that plus positive zone just over minus one. The heavy rain is located near the zero line at the south of the positive value zone of the barotropic moist potential vorticity in lower troposphere. The negative value of the baroclinc moist potential vorticity zones has a very good correspondence with the location and movement of the heavy rain.