By analysing circulation background,satellite cloud picture evolution and dynamical,thermodynamical conditions of the heavy precipitation aroused by the southwest vortex in Yunnan on August 4,2004 .The results show that the southwest vortex is very deep and baroclinic system,the southwest vortex on 500hPa is more obvious and arouses the southwest vortex on 700 hPa,the heavy precipitation is to the southwest of the southwest vortex,the heavy precipitation field has a good relation to the strong ascending motion and the positive vorticity that have stronger indication than the cyclonic circulation for the heavy precipitation and they gradually develop from the high altitude to the low altitude,mesoscale convective cloud clusters move to the south and develop into the mesoscale vortex cloud system arousing the heavy precipitation in Yunnan ,the heavy precipitation is in the negative field of（MPV1+MPV2）,both of barotropic and baroclinic instability are related to the enforcement of vortex cloud system,MPV1<0 and MPV2<0 are favorable to the heavy precipitation.