On the basis of the observed hydrological data in Qinghai Lake Basin, 1959-2000, we studied the dynamic changes of precipitation, runoff, evaporation, lake water level and the supplement of ground water of Qinghai Lake for the past forty years. Estimates revealed that a 3.32 m-drop occurred during 1959-2000, resulting an annual decrease of water quantity of 3.60×10⁸ m³. Moreover, the amount of interbasin diversion required for maintaining ecologic balance and social economic sustainable development was discussed, an annual quantity of 4.1×10⁸ m³ could meet the needs for water resources allocation optimization.