Qinghai Lake, the biggest saline lake in China, is a mountainous lake located in thenortheastrn part of the Qinghai-Tibet Plateau, and has a clsed drainage basin. Since the beginning ofthis century, the lake leve has dropped significantly and the lake m has shrunk dramatically. Waterbalance analysis showed that the water budget deficit associated with decreased runoff was the main reasonfor the falling of lake level. In order to investigate the changes of hydrlogical conditions withen the lakebasin, a catchment, model coupled with a lake thermodynamicc medel was developed to simulate the lakewater balance and thermodynamics The simulated water balance and lake level are quite consistent withobservations. This model can be used to evaluate hydrological condition changes induced by external forc-ing such as climate changes. Qinghai Lake, the biggest saline lake in China, is a mountainous lake located in thenortheastrn part of the Qinghai-Tibet Plateau, and has a clsed drainage basin. Since the beginning ofthis century, the lake leve has dropped significantly and the lake m has shrunk dramatically. Waterbalance analysis showed that the water budget deficit associated with decreased runoff was the main reason for the falling of lake level. a change in hydrological condition withen the lake basin, a catchment, model coupled with a lake thermodynamicc medel was developed to simulate the lakewater balance and thermodynamics The simulated water balance and lake level are quite consistent withobservations. This model can be used to evaluate hydrological condition changes induced by external forc-ing such as climate changes.