Abstract: Based on the ERA5 reanalysis data from European Centre for Medium-Range Weather Forecasts (ECMWF), precipitation data from National Meteorological Information Centre (NMC) and FNL analysis data from National Centers for Environmental Prediction (NCEP), the diurnal periods of heavy rain under the influence of a southwest vortex in Sichuan Basin from August 15 to 16, 2020 was compared and analyzed, the differences and similarities of the diurnal precipitation characteristics under the control of southwest vortex were discussed. The results showed that: (1) The southwest vortex enhanced at night and weakened in the daytime during the whole process, and there were significant differences in the precipitation between the nighttime rainstorm and the daytime rainstorm under the influence of the southwest vortex. The cumulative precipitation of the nightime rainstorm was 174.7 mm, while the cumulative precipitation of daytime rainstorm was 104.6 mm, and the intensity and region of the precipitation at night was significantly greater than that in the daytime. (2) Under the influence of the southwest vortex, the vertical ascending motion of the nighttime rainstorm was strong, and the updraft reached up to 150 hPa (tropopause), and the range was obviously larger than that of the daytime rainfall. (3) Under the influence of southwest vortex, there were obvious high humidity areas in nighttime precipitation and daytime precipitation, and the range expanded with time. The specific humidity intensity and air column precipitable water in the nighttime period were significantly stronger than those in the daytime period, and there was a strong water vapor flux convergence. The strong water vapor transport at night was one of the main factors that caused the precipitation to be significantly higher than that during the day.