Abstract: This paper analyzed the water vapor transportation of the heavy rainstorm in western Sichuan Basin during August 14-17, 2020 by using meteorological data from a weather station 5001 in Sichuan Province, the global data assimilation system (GDAS), and the reanalysis data from national centers for environmental predication (NCEP). The Lagrange mixed single particle orbit model (HYSPIT4) is used in the analysis. The analysis results show that the heavy rainstorm has obvious diurnal variation, and the precipitation at night is severe with 4 peaks. In the initial stage of the heavy rainstorm, the water vapor mainly came from the Bay of Bengal, and with the precipitation event continues, the water vapor from the eastern Arabian Sea, the southern Bay of Bengal and the Gulf of Thailand was continuously transported to the Sichuan Basin. According to the quantitative analysis of the contribution of different water vapor sources, the Bay of Bengal was the dominant water vapor source, followed by the Arabian Sea, and the Gulf of Thailand was the lowest.