Abstract: Based on NCEP reanalysis data and ground automatic observation station data, the causes of a flood-causing extreme rainstorm in Hunan Province from July 6 to 9 in 2019 are analyzed diagnostically from the perspective of circulation features and vortex evolution. The results show that: this extreme rainstorm was characterized by significant extremity and diurnal variation of “Xiao Xiang Night Rain”. Under the circulation background of long-term stability of subtropical high and anomalous southward shift of main circulation as well as ground cold air invasion, the strengthening and long-term maintenance of vortex caused by the asymmetric distribution of potential vorticity and continuous strong ascending motion in the middle and lower levels were the main causes of this extreme rainstorm. The strengthening period of vortex corresponded well with the strongest rainfall period, and the center of positive potential vorticity corresponded well with the center of rainstorm. In the development stage of heavy rainfall, the vertical helicity maintained the distribution characteristics of “lower positive and upper negative”, and the size of the lower positive center was consistent with the change of rainfall intensity. The heavy rain belt in the south of central Hunan and the large value center of low-level positive helicity appeared in the steep terrain area of the northern foot of the Nanling Mountains. The water vapor mainly came from the boundary layer. The occurrence time and area of the water vapor convergence center at 925 hPa were consistent with the occurrence time and falling area of the rainstorm. The two main water vapor conveyor belts were derived from the westerly airflow in the Indochina Peninsula of the Bay of Bengal and the southwest airflow in the South China Sea.