The Tibetan Plateau boasts the greatest lake group in the Asian continent, where the lakes respond sensitively to climate change. For the exorheic lakes, the area changes based on remote sensing data are insufficient to reflect their responses to climatic changes. Water balance analyses of these lakes are needed for understanding the hydrological processes of lake basins and their relationships with climate changes. In this paper, we use the hydrological and meteorological monitoring data in the Ranwu Lake Basin from April to November in 2015 to examine the relationship between water level and runoff and reconstruct flux process line according to continuous water level data. Together with the snowmelt runoff model (SRM)simulation, we analyze the water balance process and its seasonal changes of the Ranwu Lake. The result shows that the total water yield inputted into the lake during the monitoring period is about 18.49*10~8 m~3, and that the glacial melt water is about 10.06 *10~8 m~3, accounting for more than 54% of the lakes' supplies. Precipitation and evaporation of lake water surface and the lake water storage change have only slight effects on the process of lake water balance. Replenishment of the lake water is clearly seasonal as it depends on rainfall. Under the influence of southern branch of Westerlies, the Ranwu Lake area witnesses high precipitation, which is the main supply source in spring. Due to temperature rise in dummer and early autumn, a large amount of glacial melt water is a dominant factor of the water balance of this lake. With the temperature rise in the future, glacial melt water will occupy higher proportions in the total supplies of the lakes in this area. It will conduce to the speedy rise of the glacial lake level and lead to potential hazard risks.