Ecological water replenishment is an important way to maintain and improve the ecological environment of Lake Baiyangdian Watershed. In order to study the impact of ecological water replenishment on the water environment of Lake Baiyangdian Watershed, the samples of lake water, river water and groundwater were collected before and after ecological water replenishment respectively, and the hydrochemical characteristics of surface water and groundwater were analyzed. The results showed that:(1) Before and after ecological water replenishment, sodium-ion was the main cation in the hydrochemical composition of surface water and groundwater; bicarbonate radical-ion was the main anion after ecological water replenishment, and the conductivity of surface water in the south of Lake Baiyangdian was high. After ecological water replenishment, the mass concentrations of calcium-ion, magnesium-ion and bicarbonate radical-ion in surface water and groundwater increased significantly, the water conductivity decreased. (2) Before ecological water replenishment, the groundwater was Na-HCO₃ type, and the surface water was mainly Na-Cl·SO₄ and Na-Cl·HCO₃ types; After ecological water replenishment, the surface water and shallow groundwater evolved into Ca·Mg-HCO₃ type, and the hydrochemical types of deep groundwater were basically unchanged. (3) Ecological water replenishment increased the water level and water area of Lake Baiyangdian, alleviating the problem of water shortage; meanwhile, the hydrochemical composition of shallow groundwater was changed, while the deep groundwater was not affected. After ecological water replenishment, the concentrations of sodium-ion, chloridion-ion and sulfate-ion decreased significantly, while the concentrations of calcium-ion, magnesium-ion and bicarbonate radical-ion increased. In addition, the cation exchange and water rock interaction in the rising process of regional groundwater level should be paid attention to, so as to provide scientific basis for rational allocation of ecological water replenishment and improvement of ecological environment of Lake Baiyangdian watershed.