Abstract: Based on hourly precipitation data from automatic weather stations, hourly temperature data from multisource fusion, high-altitude observation data, meteorological satellite data from FY-4A, X-band dual polarization radar data, and ERA5 reanalysis data, the mesoscale characteristics and causes of a warm-sector rainstorm process in the Panxi area of southern Sichuan from 2000 BJT on October 4, 2022, to 1200 BJT on October 5, 2022 are analyzed. The results show that (1) this process manifested as a β–γ mesoscale convective system on satellite imagery, which went through three stages of developing, strengthening, and weakening in the Zemulong Village area. The mesoscale convective system of rainstorms tilts east from bottom to top, allowing the convective system to sustain itself and develop for a longer period. (2) Radar echoes in the Zemulong Village area show a strip-shaped distribution with notable back-propagation and echo training effects. The strong echo nuclei are relatively low, mostly below 4.5 km, and each nucleus weakens rapidly; thus, they exhibit a high precipitation efficiency. (3) During this process, the atmosphere is in an unstable state of dryness above and wetness below, with a low convective condensation level, lifting condensation level, and level of free convection; thus, the gas mass does not need to be lifted too strongly to move upward and cause water vapor condensation; therefore, the thunderstorm system can rapidly develop. Furthermore, weak vertical wind shear reduces the involvement of unsaturated dry cold air, weakens the entrainment rate, and improves precipitation efficiency. (4) The convergence and cyclonic flow field formed by the blocking effect of the terrain on the northwest edge of Panzhihua is the main factor for generating continuous convection and forming training effects; thus, extreme precipitation occurs in the Zemulong Village area. Meanwhile, in the Guanyintang Village, the southwest low-level jet stream is nearly orthogonal to the northeast mountains, which forces the airflow to rise and form heavy precipitation in the low-lying areas at the front of the mountain. Owing to the southwest low-level jet, the convergence of wind speeds caused by terrain and terrain circulation caused by terrain temperature gradient also promote the formation of heavy precipitation.