In order to quantitatively evaluate the influence of Xia'an Reservoir on the downstream hydrological regime, the runoff series of Baizhi'ao hydrological station in the Yong'an River from 1980 to 2020 were used to analyze the annual distribution change of runoff of the Yong'an River before and after the operation of the reservoir by using the nonuniformity coefficient, complete regulation coefficient and concentration degree, and the RVA method was used to evaluate the overall change degree of hydrology. The results show that the Xia'an Reservoir has an obvious effect of storing abundant water and replenishing dry water, and the annual distribution of runoff tends to be uniform. While presenting flood control benefits, the operation of the reservoir also has a moderate change in the hydrological regime of the Yong'an River, with a change degree of 42.82%, which has a potential impact on the health of the aquatic ecosystem in the basin.