Abstract: Under the theory framework of potential vorticity (PV) conservation, the evolution of a southwest vortex which induced severe rainstorms happening in Guangxi province and the Mesoscale Convective Systems (MCSs) companied with this vortex was analyzed based on various observation data and numerical simulation results. According to the synthetic analysis of synoptic weather systems, the low vortex developed rather rapidly in the beginning, and the southwest low level jet strengthened distinctly which created a suitable environment for convection. In light of the diagnosis of PV, it is revealed that the mass transporting effect by the MCSs activities resulted in the distribution of positive PV anomaly in the upper level and negative PV anomaly in the low level, and the positive feedback between the convective heating and the positive PV anomaly explained the quick deepening of low level vortex. During the stable developing stage of the southwest vortex, the mesoscale shear line and low level jet formed in the vortex made an important impact on the evolution of precipitation and MCSs. On the night of 12 Jun, due to the influencing of positive PV anomaly increasing in the upper level, the coupling effects by the upper and low level systems lead to rapid growing of MCSs which struck Liuzhou severely. The results by reversing PV indicated that the evolving of the southwest vortex is a quasibalanced dynamical process on the synoptic scale, the balanced flow can represent the main structure of the southwest vortex, and the nonbalanced flow is able to be used for revealing the details of the MCSs development in the southwest vortex more clearly.