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Using multi-source observation data and ERA5 reanalysis data, a rainstorm that triggers a record-breaking flood in Qijiang Basin (QB) from June 19 to 22, 2020, is analysed. The nocturnal enhancement mechanism of precipitation brought by Southwest Vortex is hereafter studied. The results show that: (1) The rainstorm, exhibiting significant daily variation, intensified during the night and early morning. The whole process was divided into three stages. At the third stage, the MCSs showed lower Blackbody Brightness Temperature (TBB), moved and propagated more slowly, and resulted in stronger precipitation and convection with longer duration. (2) Under the background of a weak trough in the westerly to the north of the Western Pacific Subtropical High, the rainstorm was caused by a strong Southwest Vortex, a stable low-level jet, and a convergence line on the surface. At the stage of convection initiation, an unstable environment with high temperature and humidity was formed near the convergence line. Meanwhile, the northerly wind on the surface was forced to lift by terrain, then joined the updraft of basin-plateau secondary vertical circulation, and later triggered convection. (3) The low-level ageostrophic wind above Sichuan Basin (SB) and northern Yunnan-Guizhou Plateau (YGP) rotated clockwise every day, both turned southerly and strengthened at night, and was stronger above YGP than above SB. The nocturnal low-level wind, being sub-geostrophic above SB and super-geostrophic above YGP, caused warm shear and flow convergence. Additionally, the water vapour flux transported by ageostrophic wind was negative during the day and positive at night, while the water vapour flux transported by geostrophic wind remained positive all day, resulting in a significant positive increase in the water vapour flux transported by actual wind at night. The above daily variations were conducive to water vapour accumulation and flow convergence, which eventually led to the intensification of precipitation. (4) The advection and ageostrophic terms in the momentum balance equation became positive or increased at night, and were greater above YGP than above SB. The horizontal wind on YGP therefore intensified, wind speed gradient and convergence were formed as a result. The ageostrophic effect played an important role in the nocturnal enhancement mechanism of precipitation.