Abstract: Precipitation exerts a strong control on runoff in the Dadu River Basin, and the accuracy of basin-scale precipitation forecasts is crucial for water resources regulation and efficient utilization. In particular, precipitation over the upper reaches has a greater impact on runoff in the middle and lower reaches of the basin. However, studies focusing on precipitation processes in the upper Dadu River Basin remain limited, which constrains improvements in precipitation forecasting skill. This study examines a precipitation event that occurred on 20 June 2020 over the upper reaches of the Dadu River Basin on the western Sichuan Plateau, characterized by weak upper-level divergence forcing. Based on synoptic circulation analysis, a mesoscale numerical model is used to conduct high-resolution simulations of this heavy rainfall event. After successfully reproducing the main features of the rainfall process, the simulated results are further employed to analyze the evolution of dynamic factors associated with the event. The heavy rainfall was jointly influenced by mid–low-level weather systems and complex topography. During the precipitation period, the Dadu River Basin was controlled by the center of the South Asian High, with no pronounced upper-level divergent outflow, while a weakly developed westerly trough at 500 hPa moved eastward, providing a background of weak mid-level ascent. In the initial stage of precipitation, easterly flow over the northern part of the basin at 600 hPa was lifted by the Daxue Mountains, triggering rainfall. Subsequently, southeastward warm and moist airflow transported by a low-level southwest vortex entered the middle and lower reaches of the basin, leading to further development and eastward propagation of precipitation. Another ascent center formed from the middle–lower Dadu River valley to the Qionglai Mountains, causing the rain belt to split into two precipitation centers that later weakened while moving eastward. The analysis indicates that the vertical integrals and horizontal distributions of three dynamic factors—moist thermodynamic advection parameters, thermal helicity, and the vorticity–divergence gradient—correspond well with the locations of heavy rainfall. Their spatial patterns and evolution are consistent with the movement and development of the rainstorm, effectively indicating the evolution and dynamics of heavy rainfall systems over the complex terrain of the upper Dadu River Basin. The results provide useful references for precipitation forecasting over the steep terrain of the western Sichuan Plateau and the Dadu River Basin, and contribute to the development and utilization of water resources in western China.