<p>ERA5 reanalysis data, GPM satellite precipitation products, and radar mosaic combination reflectivity (RMCR) data were used to analyze the influence mechanism of mesoscale gravity waves (MGWs) on the Southwest vortex (SWV) and its precipitation during a rainstorm process that occurred in the Sichuan Basin (SCB) from 30 Jun to 1 Jul 2020. The rainstorm was affected by the SWV, upper trough, and the MGW with a wavelength of 150–200 km and a period of 4–5 h. The MGW was initiated by the combined effects of the shear instability mechanism, the non-equilibrium flow mechanism, and steep slope terrain. The updraft of the SWV was coupled with the updraft phase of the MGW, resulting in transient, periodic deep convection. The large-value area of equivalent potential temperature corresponding to water vapor convergence caused by the eastward-moving MGW resulted in the enhancement of equivalent potential temperature in the upper troposphere. A critical layer and strong vertical wind shear formed in the upper troposphere caused the reflection of the MGW, promoting downward energy and phase propagation of the MGW. Meanwhile, the strong vertical wind shear in the lower troposphere also led to the reflection of the MGW, transporting energy upward in front of the eastward propagation path of the MGW, thereby enhancing convection and precipitation in the front of the eastward-moving SWV. A comparative analysis of two rainstorm cases indicates that when the MGW is phase-coupled with the SWV, cold cloud processes and warm cloud processes jointly dominate rainstorm occurrence.</p>