Abstract: Using the conventional surface and upper-air meteorological observation data, NCEP reanalysis data and GDAS data, combined with HYSPLIT model, the circulation situation and water vapor characteristics of the flood-causing rainstorm weather process on August 21, 2023 in Liangshan Prefecture were analyzed. The results show that short-wave trough, shear line, surface cold air and strong unstable energy were the favorable conditions for the rainstorm. The water vapor at 700 hPa mainly came from the southeast airflow, the water vapor at 600 hPa mainly came from the southwest or southerly airflow, and the water vapor at 500 hPa mainly came from the north, thus forming an unstable stratification of “upper cold and lower warm”, which was conducive to triggering convective weather. With HYSPLIT model, the further cluster analysis of the tracking results showed the water vapor in the middle and lower levels (700~600 hPa) mainly came from the local area of Liangshan Prefecture and its surrounding areas, followed by Bay of Bengal and its surrounding countries, the water vapor contribution rate of the two was more than 90%. The water vapor at 500 hPa mainly came from Bay of Bengal and its surrounding countries and the eastern side of the Tibetan Plateau, the contribution rate of water vapor was 79%. The contribution of water vapor to the rainstorm process in Liangshan and its surrounding areas was higher at the lower level. The MPV showed that there were configurations of MPV1 < 0 and MPV2 > 0 in the lower layer during the process of heavy rain, and the regions of MPV1 < 0 and MPV2 > 0 were in good correspondence with the heavy rain area.