With conventional meteorological data as well as NCEP 1°×1° reanalysis data, the processes of the super typhoon Rammasun(1409) and Mujigae (1522) are diagnosed and compared. The result indicates that the re-strengthening of the typhoon Mujigae after its landing and denaturing was due to intense development of a frontal cyclone, which was incarnated by downwards moving of the up-per level potiential voticity, the tropical low pressure circulation and the low-level front. But in the process of the denaturing of the typhoon Rammasun, there was no downwards moving of the up-per level potential voticity, no low-level front, and the atmospheric slanting piezotropy was weaker. So the mutual influence of downwards moving of the up-per level potential voticity, the tropical low-pressure circulation and the low-level front did not exist. And that is the reason why more severer rainstorm in south China caused by Mujigae than Rammasun. In the two processes, the negative MPV (Moist Potential Vorticity) center at 925hPa agrees well with the severe precipitation region located the side of warm and moist airflow 110 kilometers away from the negative MPV center, whose size can reflect the precipitation. And the size of the negative MPV center of Mujigae are larger than that of Rammasun. The positive MPV center and the turning point of θse equivalent surface are indexes for predicting the strong precipitation center. So the MPV at low levels can be used to forecast falling areas of rainstorm 6 hours in advance.