Taking the test section of Doumen Reservoir in Shaanxi Province as an example, the permeability coefficient of the sediment at the bottom of the reservoir was calculated by field sampling and laboratory test. The leakage amount and leakage strength of the reservoir area were obtained by analytical solution, and the groundwater flow field changes under reservoir water storage conditions was simulated by numerical method. The results showed that the permeability coefficient of the sediment at the bottom of the reservoir and the leakage strength were 10-3 m/d and 14.97 mm/a, respectively; and the permeability coefficient and leakage strength in the northeastern part of the reservoir area were relative large. The leakage in the reservoir area was about 7070.05 m3/a, accounting for 4.6‰ of the total storage capacity, and the leakage change was more affected by the precipitation. After five years of reservoir impoundment, the regional groundwater level rises 0.1~0.5 m, and the groundwater in the northeast of the reservoir area. The force gradient significantly increased and the groundwater flow direction deflected.