By using the theory of moist potential vorticity(MPV),the characteristics of MPV are analyzed in the process of two rainstorms caused by typhoon or weaken typhoon circulation in Shandong Province. The results show that the two heavy rain events occurred in the very stiff and dense section of moist isentropes, which can lead to the development of moist slantwise vorticity. MPV1<0 and MPV2>0 in the low troposphere reflect the development of convective and baroclinic instability . The downward spread of high from the upper troposphere is also of an advantage to the release and deposit of instability energy and the results in the amplification of rainstorm.