By using the mesoscale numerical model MM5 V3.6, a successful simulati on is made for heavy rain event in the central and southern parts of Shandong Pr ovince on September 19-21, 2005. And the moist potential vorticity of the heavy rain is analyzed by using high resolution simulation result. The results indicat e that the stiff moist isentopes excite the development of moist slantwise vorti city and occurrence of heavy rain. A diagnosis of moisture potential vorticity i s made to show its temporal and spatial mesoscale characteristics. The results s how that the development of heavy rain is closely related to the temporal and sp atial variations of moisture potential vorticity. The heavy rain occurs in the i soline dense section of the negative MPV1 and positive MPV2, and the rain center lies in the convective instable section in the front of the negative MPV1 cente r at 850hPa.