Abstract: Based on data of conventional meteorological observation, automatic station precipitation and Doppler weather radar, the causes of the regional extraordinary rainstorm on August 8, 2021 in Dazhou are analyzed. The results show that: (1) The coupling of upper trough and deep vortex system provided strong dynamic conditions for the extraordinary rainstorm; the southwest jet stream provided sufficient water vapor and energy for heavy precipitation; the backflow cold air of the boundary layer and the ground convergence line provided a triggering mechanism for the occurrence of strong convection. (2) Strong convective echo, slow moving speed, stable position and the train effect were the direct causes of the extraordinary rainstorm; the upwind area had a significant increase effect on the heavy precipitation, and its location and occurrence time corresponded well with the rainfall area and time interval of heavy precipitation; the southerly jet stream at low level and radial velocity convergence at low level provided water vapor and dynamic conditions for heavy precipitation. (3) High VIL value in midsummer had a good indication action on the extraordinary rainstorm.