The characteristics of moist potential vorticity are analyzed during two heavy rainfall and two hail events in Northern Akesu. The results show that the stiff moist isentropes excite the development of moist slantwise vorticity and occurrence of heavy rainfall and hail. When MPV1<0 and MPV20 at 850hPa and MPV1>0 and MPV2<0 at 700hPa,sudden heavy rainfall probably happen. The order of magnitudes of MPV1 and MPV2 is equivalent. When hail occurred, there are MPV1>0 and MPV2<0 at 850hPa. Because of the difference of affective systems, there are different distributions for MPV1 and MPV2 at 700hPa, but reverse of MPV1 and MPV2 lead the development of slantwise vorticity.