 Only by understanding forest hydrological effect accurately and objectively can we get right direction from these principles． This paper proposes using systematic viewpoint to deal with the relationship between forest and water． Comprehensive adjustment function of forest and the relationship between forest and hydrologic cycle are stated and analyzed systematically depen ding on the long term research results of fixed ecological stations throughout China and similar research projects． It raises that in the development of western regions of China， scientific allotment of water resources should be considered in construction of forest grass vegetations and all industrial development should be moderate．