The potential vorticity (PV) on isentropic surface and moist potential vorticity (MPV) diagnoses in the mid-Juneof 2008during Guangxi flashflood rainstorm process under the influence of mesoscale vortex (Guangxi vortex for short) are carried out using the high-resolution model data. The results show that the location and strength of positive PVon low-value isentropic surface reflect the area and intensity of heavy rain; dry and cold high-value isentropic potential vorticity (IPV) on high-latitude and warm and moist high-valueIPVon low-latitude are, respectively transported to vortex area in Guangxi by westerlieson isentropic surface, resulting in the maintenance of high-value IPVin Guangxi vortex. While on the high-value isentropic surface, there is transportation of southern IPV to its downstream, which is beneficial to the IPVgrowth of Guangxi vortex which is inits downstream, resulting in strong development of Guangxi vortex and  precipitation enhancement. The analysis onthe MPVconservation principle shows that continuous heavy precipitation appeared in the convectively unstable area in front of high-value isentropic moist potential vorticity. The convective stability decreases with the going down of cold airinmid- and upper-leveltroposphere on north and south sides of rain area, making the corresponding cyclonic vorticity increase; while the warm and moist air climbs along dry and cold air, and there is strong convergence between the warm air and the cold onegliding down the isentropic surface, making the vorticity sharply increase, and with the uplifting effect forced by terrain, strong vertical ascending motion is formed and the vortex strongly develops. The distributions of isobaric surface MPVand its components show that negative MPVin the lower-leveltroposphere can denote the heavy precipitation, and there is convective instability and conditional symmetric instability during heavy precipitation period.