From 26 to 28 July 2023, the Sichuan Basin experienced a prolonged and widespread heavy rainfall event. This process, characterised as the most intense rainfall since the onset of the flood season 2023 in the basin, led to significant localised flooding disasters. To analyse the characteristics and evolution of the southwest vortex responsible for the heavy rainfall in the Sichuan Basin, the study utilises ERA5 reanalysis data, GPM surface precipitation data, and FY-2G cloud-top equivalent blackbody temperature data, applying synoptic diagnostic methods and the vorticity budget equation. The conclusions are as follows: (1) The southwest vortex belonged to the non-moving-out type, and in addition to being induced and coupled with the plateau vortex, the typhoon also promoted the development of the southwest vortex by transporting vapour over a long distance and changing the wind pressure field structure. (2) The mesoscale system activity experienced five stages: mesoscale convergence disturbance, southwest vortex generation and development, intensity weakening, redevelopment and intensification, and weakening filling. The scope and intensity of precipitation during the redevelopment stage were greater than those during the initial development stage. (3) In the redevelopment stage, the southwest vortex system became deeper, the height of the positive vorticity column increased to 400 hPa, and the range expanded along the height to the northwest, with the feature of “convergence at low level and divergence at high level” significantly enhanced. (4) The water vapour during the initial development period was mainly contributed by the humid airflow from the Bay of Bengal. During the redevelopment period, it was transported westward from the South China Sea to the Sichuan Basin by the typhoon, and the southwest vortex region maintained a high humidity state. The asymmetry of the warm wet structure was strengthened, and the baroclinicity was strong. The degree of convergence and divergence of water vapour corresponded to the location of the southwest vortex and precipitation intensity. (5) The lower-level positive vorticity budget of the two development stages was mainly contributed by the stretching term, the vertical transport term, and the twisting term.