Abstract: Diagnostic analysis on a case of southwest vortex over the eastern Sichuan basin in 8~10 July 1989 reveals its three-dimensional structure in mature stage. The results show that the southwest vortex is a mesoscale cyclone and depression throughout the troposphere. Its cyclonic vorticity could vertically extend to 100 hPa, and center axis is nearly vertical. The distributions of momentum, stratification and vertical motion present asymmetry in the vortex.