Abstract: Using conventional observation data, NCEP reanalysis data (FNL) and synoptic methods, a diagnostic analysis on a rainstorm that occurred in the Sichuan Basin on October 4, 2021 was conducted. The results show that the moisture for the rainstorm primarily originated from the South China Sea and the Bay of Bengal. The formation and intensification of the rainstorm were closely related to variations in the moisture flux convergence zone. The severe weather event resulted from the combined influence of multiple factors, including the Western Pacific Subtropical High, an upper-level trough, a southwest vortex, a low-level shear line, a low-level jet and topographic lifting. The configuration of vorticity and divergence in the lower and upper levels over the heavy rain area triggered strong upward motion. The upward motion transported moisture and heat from the lower levels to the upper levels, facilitating the development of precipitation.