Abstract: Many studies have been conducted on the Ya’an rainstorm in Sichuan Province, which is caused by the unique bell mouth terrain. Research on the warm-area and nocturnal rainstorms in this region should be strengthened as part of the promotion of the urgent demands of operational forecasting and disaster prevention and reduction. The dynamic and thermal structure evolution, trigger mechanism, and topographic influence of the torrential rain process that occurred in Lushan county, Ya’an, Sichuan Province, on August 10, 2020, were analyzed using the ERA5 reanalysis data, combined with the ground-encrypted observation data and the three-source fusion near real-time precipitation data provided by the Meteorological Information Center of the China Meteorological Administration. This research reveals the configuration of water vapor, dynamic and thermal structure evolution, and triggering mechanism of torrential rain in a warm location under the background of weak synoptic forcing and special topography. The results are as follows: (1) This severe rain process was associated with a warm rainstorm of southeast winds, with no clear influence system at 500 hPa and no jet stream at a lower level. The combined action of the western Pacific subtropical high moving westward, the strengthening of warm and wet airflow from the southeast, and the weak vortex convergence airflow at 850 hPa affected by Ya’an’s “windward slope” and “bell mouth” topography and Lushan’s southwest “˄” type canyon topography caused this brief and intense precipitation. (2) From the beginning to the peak of precipitation, the uplift velocity, cyclonic vorticity, and horizontal convergence caused by boundary layer topography were always superimposed with systematic vertical upward movement, vorticity, and divergence, which enhanced low-level convergence, intensified vertical upward movement, and promoted precipitation intensification. (3) The rainy region experienced increased convective instability due to the differential advection of θ_se. The weak cold advection of θ_se in 500 hPa was also one of the triggering elements of rain in warm regions. In a high-energy and high-humidity environment with convection suppression, the energy was zero. The initial convection was caused by cold orographic advection in the evening and dispersed along the 1500 m terrain line. A γ mesoscale convergence line was formed on the west side of the “˄” type valley and maintained by upstream heavy precipitation, which caused thunderstorm cold pool outflow and mountain wind. After rapidly descending the mountain, the cold pool also remained near the mountains on the east side of the “˄” type canyon, thereby forming a strong temperature gradient. These factors triggered and maintained the torrential rain in Lushan at night.