Abstract: Based on the hourly precipitation data from automatic weather stations, FY-2G TBB data, and ERA5 reanalysis data, dynamic analysis and numerical experiments are performed for a warm-sector mountain rainstorm event in the western Sichuan Basin on 23 July 2017. The results showed that: (1) The warm-sector mountain torrential rainstorm occurred at the edge of the West Pacific subtropical high under the background of weak synoptic forcing. The high temperature and high energy in the western Sichuan Basin and the southeast wind intruding into the basin, lifted by Longmen Mountain, induced this rainstorm. (2) The conversion of the mountain–plain circulation is the reason for the intensification of the rainstorm and the reorganization of the mesoscale convective cloud clusters. (3) Moreover, the mountain–plain circulation uplifts the background wind that causes the upward sloping motion. Further study of the results of the numerical simulation showed that the mountain–plain circulation is driven by near-surface thermal perturbation. In the daytime, a positive virtual temperature disturbance area on the hillside of the western Sichuan Basin is detected, and a plain-to-mountain flow is developed. After sunset, the distribution of the virtual temperature disturbance on the mountain and plain is reversed. Therefore, the mountain–plain circulation shifts from mountain to plain. When the model surface heat source is removed, the near-surface thermal perturbation tends to disappear, and the mountain–plain circulation does not form in the western Sichuan Basin. Consequently, the convergence area related to the mountain–plain circulation dissipates, resulting in the decline of the simulated cumulative precipitation and the disappearance of the heavy precipitation center.