Abstract: A regional warm area rainstorm occurred in the western Sichuan Basin from August 1 to 2, 2018. The circulation background, mesoscale conditions and trigger mechanism were analyzed by using the data of conventional meteorological observation, regional automatic stations, satellite cloud images and radar products. The results show that the eastward moving plateau vortex triggers the rainstorm weather, which increases the low-level vorticity and forms the center of cyclonic vortex. The coupling of plateau vortex and southwest vortex strengthens the vertical upward movement in the western basin. The low-level water vapor and unstable energy are forced to rise on the windward slope, triggering convective precipitation and increasing precipitation, resulting in stronger precipitation in southwest basin than in northwest basin. The characteristics of high humidity environment, deep warm cloud and medium-strong and narrow CAPE formed high precipitation efficiency. The strong precipitation period corresponds to the strong cloud development period, and the formation of convergent wind field and leeward area is conducive to the long-term maintenance of strong echo.