It is widely accepted that the lake water level will descend in the northwest of China in the near future. Because most of lakes in northwestern China belong to closed basins, the descent of water level often results the condensation and salination of the water, bringing about a series of environmental problems to the lakes and the catchment areas. A calculation based on the analysis of climate, runoff, water balance and water level in Daihai Lake basin shows that the total fall-down of water level is 3.85 m during 1955 to 1995, among which human factor occupies a drop of 3.17 m. It is very clear that human activities give very strong impact on the water balance of Daihai Lake.