Plateau lakes are extremely sensitive to climate change, and climate change can be truly reflectedthrough the variation of lakes. Based on multi-source and multi-date digital remote sensing images, topograph?ic maps and digital elevation model (DEM) data, the lake boundaries were extracted with geographic informa?tion system and remote sensing technologies. Combined with other related research literatures, the area andboundary variation of Ulan Ul Lake in Hoh Xi1 Region and the correlation between such variations and thenatural elements (annual mean temperature, annual precipitation, etc.) were analyzed from 1976 to 2012 inthis study, and then the causes for such variations were also discussed through the angle of water balance ofthe lake. The results showed that Ulan Ul Lake had expanded on the whole during the period from 1976 to2012, but experienced shrinkage first before the expansion. The lake was in a shrinkage during the periodfrom 1976 to 1994 at a shrinkage rate of 3.12 km~2/a, with its area varied from 555.97 km~2 in 1976 to 496.50km~2 in 1994. A rapid expansion has started since 1998 with an expansion rate of 10.36 km~2/a, and the lake areaexpanded from 499.83 km2 in 1998 to 655.25 km2 in 2012. In terms of space, the area variation of Ulan UlLake was mainly in the southern part where rives flow into the lake. The trends of meteorological factors overthe Ulan Ul Lake basin from 1976 to 2012 showed the characteristics of increasing annual average tempera?ture and increasing annual precipitation. There were a positive correlation between the annual mean tempera?ture and the area of the lake. Comprehensive analysis revealed that the area variation of Ulan Ul Lake since1998 was mainly caused by two factors, one was the increasing annual precipitation, and the other was the ris?ing annual mean temperature, which leaded to the thawing of permafrost, ice and snow. Based on analysis ofwater balance of the lake, annual precipitation directly falling on lake, annual runoff deriving from the annualprecipitation in the basin, and annual thawing water were the three main recharge resources of Ulan Ul Lake,respectively accounting for about 23.3%、43.7% and 33.0% of the total annual recharge quantity of the lake.