Abstract: The forecast of heavy rain induced by vortex is not only determined by the vortex movement track, but also closely related to its structure and revolution. Based on NCEP/NCAR 1°×1° 6-hourly reanalysis data, the structure characteristics of southwest vortex and the mechanism of heavy rain in July 2008 are analyzed. The results show that the producing process of southwest vortex includes the coupling and induction of Tibetan Plateau vortex,and the generation and development of southwest vortex are related to the disturbance transmits of dry potential vorticity downwards to the lower troposphere. The confrontation of cold air from middle-high latitude and warm-wet air along the edge of the sub-tropic high strengthens baroclinicity of the system, which makes the potential vorticity column and positive vorticity column over the vortex center incline westward with height. The mature southwest vortex is with meso-scale asymmetric significantly baroclinic structure. The positive vorticity advection in the mid-troposhere is an important factor which makes southwest vortex develop and move. The condensation heating make southwest vortex develop and enhance the precipitation