With the Manas Lake in Xinjiang as the study area,by using eight remote sensing images from 1972 to 2014,and on the basis of extracting the information of lake water,the authors analyzed the evolution of the Manas Lake area in the past forty years. The results are as follows: The Manas Lake showed a significantincrease - decrease - increase - decreasetrend of change from 1972 to 2014,and the barycenter of water area was migrating in southwest direction. Lake had dried up from 1972 to 1999,then it restored the largest water area (248.69 km~2) in 2000,and after that it experienced two shrinking periods that happened in 20002008 and 20112014 respectively. Calculation result of area variation amplitude and dynamic degree shows that the shrinking period of the water area was shortened and the shrinking speed increased. In the past forty years or so,the change trend of lake water area was not consistent with the change trend of the Manas River. The authors point out that climate change has little effect on the change of the water area of the Manas Lake,but has an intimate relationship with extraordinary flood caused by extremely high temperature and extreme precipitation,and that human activities in the basin constitute the main reason for the evolution of the Manas Lake,which causes the declining of the function of supplying water resources to the lower reaches.