The impact of regional environmental status, development trend and reservoir operation in key water control project area on the environment is an important part of the environmental impact assessment work. This paper analyzed the water movement and supply relationship of forest and grass area at Kirgiz River downstream , calculated the change amount of groundwater recharge and depth, predicted growth conditions of natural vegetation after reservoir construction. The result can provide a scientific basis for project decision-making and engineering environmental management. Through establishing the regional water balance model Before and after construction of the project, the paper simulated surface water and groundwater water distribution of the area,circulation relations and balance change.Through monitoring study area groundwater depth data ,it also simulated the situation of groundwater level changes of forest and grass area before and after the project was completed around the river end,and analyzed the influence of groundwater level changes on riparian forest and grass because of the construction of project. The result indicated that the river surface runoff supplies groundwater,the water surface line of river is higher than groundwater level during the period of runoff, the relationship of river surface runoff and groundwater is that surface water recharges groundwater, during Shut-off period the groundwater level is higher than the riverbed elevation, groundwater of lateral penetration supplies surface water.