Abstract: This paper develops a water balance equation for the Qinghai Lake using its water stage and meteorology observations for the period of 1961 to 2015, estimates its annual precipitation, evaporation and inflow, and analyzes the time trends of its water balance components. Regression analysis is used to examine the contribution rates of different factors to the time variations in its water stage. The results show the water stage in the lake decreased before 2005 and since then has risen evidently. Its annual precipitation is 14.6×10~8 m~3, annual inflow runoff 24.4×10~8 m~3, and annual evaporation 40.1×10~8 m~3. In the period of 1961 to 2015, precipitation on the lake showed an increasing trend; its inflow runoff has increased significantly since 2003 and evaporation decreased since 2000. Inflow increase and precipitation increase together with reduced evaporation was the cause of lake stage rising after 2005,with the inflow runoff contributing the largest proportion of 38.2% to the stage variations.