Abstract: Torrential rain with the greatest social impact in Chongqing in the year of 2021 occurred in the central and eastern parts of Sichuan basin from August 7 to August 8. Using multi-source observation and ERA5 reanalysis data, the characteristics of mesoscale convective systems during the heavy rain process were analyzed. The heavy rainfall occurred under the background of the warm Southwest Vortex induced by the trough moving into the Sichuan basin, and had evident characteristics of stages, bounds, and extremes. The heavy rainfall formed successively in the southeast of the Southwest Vortex center and near the boundary layer convergence line in the east and south of the Southwest Vortex. The heavy rainfall of each stage was produced by mesoscale-β convective systems which moved slowly and maintained for 3~6 h. Warm and humid instability and weak vertical wind shear provided favorable environment for the development of mesoscale convective systems. Vorticity analysis showed that the development of the Southwest Vortex was mainly due to the low-level convergence and vertical vorticity transport. However, the development of positive vorticity in the rainstorm area was not exactly the same as that of the Southwest Vortex, and the horizontal vorticity tilting effect was relatively significant. The positive vorticity of the heavy rainstorm area in the first stage was mainly due to the significant low-level convergence, vertical vorticity transport, and horizontal vorticity tilting effect near the Southwest Vortex center in the middle and low level troposphere. The positive vorticity of the heavy rainstorm area in the second and third stages were mainly due to the horizontal vorticity tilting effect above the boundary layer and the convergence in the boundary layer. The boundary convergence triggered mesoscale convective activities in the warm and humid atmosphere, which promoted the heavy rain in the second and the third stages.