In order to investigate the reason why the deterioration of ecological environment surrounding Hulun Lake result from the decrease of water level, the paper took Hulun Lake as a research object,and used Landsat ETM + and OLI image information to obtain the situation of area change of the lake.It divided 1960-2013 into four periods and used water balance method to analyze the main factors that affect water quantityin the lake. The results showed that the influence factors and degree are different in different periods, the runoff value R were 0.42，0.93，0.6，0.86，among them,the maximum factor value in each period, the area reduced 213 km2 and the water level decreased 3.35 m. Comprehensive discussed above, the runoff of input lake is the main control factor of water quantity in Hulun Lake, and the sustanble shrinking of water area is mainly affected by climate change and human activities.