With the momentum diagnosis method, an analysis is made of the interaction between plateau vortex and southwest vortex during a heavy rain event in Sichuan on 20 to 22 July 2008. The results show that when the latitudinal distance between plateau vortex center and southwest vortex center was 5°, the updrafts in both vortexes were confined within 500 hPa; when moved eastward and coupled longitudinally, they both developed, with the updraft of the southwest vortex center reached 300 hPa and that of the plateau vortex center reached 200 hPa. The southwest vortex restricted the development of the plateau vortex in some way at the early stage when it appeared in the lower atmosphere. When they were coupled longitudinally, the vertical difference caused by the different vorticity advections between the upper and lower levels excitated the updrafts under 500 hPa and strengthened cyclone vorticity, and then the positive vorticity at 500 hPa and 700 hPa increased by two times The change rate of positive vorticity facilitated the plateau vortex to move eastward. After coupled longitudinally, the interaction of the plateau vortex and basin vortex strengthened the updrafts. As a result, the plateau vortex and southwest vortex influenced each other and developed together.