Abstract: Based on the rainfall data of national basic stations and regional automatic stations, ERA5 reanalysis data and FY-4A satellite TBB data, a rainstorm in the lower reaches of Jinsha River in September 2020 was analyzed by diagnostic analysis and numerical simulation. The results showed that the sudden rainstorm occurred under the weather background of cold air moving southward and plateau convection systemmoving eastward, and the sufficient water vapor condition and unstable layer in the early stage in Jinsha River Basin provided favorable environment for the occurrence of rainstorm. When the cold air moved southward, the convergence of warm and cold air kept the plateau convective clouds moving eastward, which led to the occurrence of this sudden rainstorm. The numerical simulation further revealed that when the cold air reached the north of the basin, the cold air merged with the southwest flow and the easterly flow of Guizhou vortex in the northwest of the basin, which resulted in precipitation. And when the cold air invaded southward near the Baihetan dam area, the shallow cold air entered the rainstorm area along the Jinsha River valley, forcimg the uplift of the southwest airflow, formig a vertical circulation circle with positive feedback effect on the development of the rainstorm, which further strengthened the rainstorm.