By using Barnes bandpass filter and mesoscale characteristic analysis method of unbalance dynamical forcing, the activity features of a Southwestern Vortex (SWV) producing severe torrential rain on 9 Sept. 2007 in Sichuan were discussed . The case study not only enables us to know that in SWV there might be a deep m idβ scale vortex with its axes westerly declining but also shows that S WV had a typical vertical structure of heavy rain, namely, it has strong converg ence in the lower level and divergence in the upper, and this kind of structure enhanced till to its top stage before wearing off. During this period, the rain area tended to approach to the center of SWV. In the point of air unbalance and at the early stage of this heavy rain process, the air had been in relatively unbalanced state. This unbalance in lower level became strong er and stronger as time goes, which was favorable for development of the mid β vortex. While heavy rain was at its largest stage, the air had changed from unbalanced state (U＜0) into quasibalanced state, U≈0.