Abstract: A large-scale rainstorm occurred in southeastern Shaanxi on April 23, 2021, with a number of 18 stations' daily rainfall being exceeded their daily rain extreme value in April, which was relatively rare. The conventional upper air and surface observational data, and ERA5 reanalysis data (0.25°×0.25°) were used to diagnose the causes and extremes of the rainstorm event. The results showed that the positive vorticity advection brought by the strong southwestern airflow in front of the 500 hPa plateau trough was conducive to the enhancement of the low-level rapid flow and the development and the maintenance of the low vortex. There were two strong water vapor conveyor belts in this event, one was the southerly water vapor conveyor belt, and the other was the easterly water vapor conveyor belt outside the tropical cyclone. The 700 hPa shear line and 850 hPa southwest low vortex brought significant water vapor convergence to southern Shaanxi, and the continuous abundant water vapor supply and convergence made the strong rainfall sustained for a long time, which led to heavy rainfall. There was a significant frontal inversion on the sounding map, the frontal region was tilted from south to north, the frontal generation was mainly contributed by the convergence and horizontal deformation terms, and the horizontal deformation term was dominated by the telescopic variability term. The southwestern warm and humid airflow climbed along the lower cold cushion, the wind speed and wind shear in the middle and lower atmosphere as well as the frontal generation caused strong water vapor convergence and uplift, at the same time, the forced uplift of the southerly rapids along the windward slopes of the Qinling and Daba mountains enhanced the precipitation finally. The standardized anomaly (SD) diagnosis showed that the anomalously strong southerly water vapor transport at 700 hPa and 850 hPa which were over 3σ and the anomalously strong easterly water vapor transport at 850 hPa which was over 3σ caused the anomalously strong water vapor content in the whole layer in Shaanxi, at the same time, the mid-level convergence combined with the vertical circulation caused by the frontal generation made the upward movement of southeastern Shaanxi relatively stronger than the climate state anomaly 3σ, which caused the extreme rainstorm process in Shaanxi in April.