Lake Qehan, an inland closed lake located in the arid and semi-arid area and the marginal belt of southeast monsoon in Inner Mongolia, is very sensitive to climatic change,but little work has been conducted in this lake. This study aimed to investigate the lake volume, area and level variation and identify the causes for the lake fluctuation during the past 50 years, based on 19 MSS/TM/ETM+ images and two aerial photographs from 1958 to 2010 and the meteorological and hydrological data of three representative meteorological stations and one hydrological station around the lake basin from 1955 to 2010. The results showed that Lake Qehan has experienced severe shrink and environmental depravation over the recent 50 years, and human activities was not the main factors of lake shrunk,but the increasingly warm and dry climate was the principal cause of the reduced lake area and volume. The mean annual temperature increased 2.5℃ and the annual precipitation declined about 36.6 mm during 1955-2010. While the lake volume decreased 66.9% (from 124.1×10⁶m³ to 41.1×10⁶m³), by an average reduction rate of 2×10⁶m³/a, the lake area reduced from 105.3 km²in 1958 to 28.1 km² in 2010 by an average reduction rate of 1.8 km²/a. The variation of lake volume was highly negatively correlated to temperature and evaporation, the correlation coefficients were -0.689 and -0.546,respectively. Lake Qehan has two parts, the west one and the east one, there was a significant lake-level decline in west lake by 7.6 m(from 1016.6m to 1009 m a.s.l.), and it has dried up in 2002, and exposed extensive expanses of former lakebed and has created salty surface, which lead to saline dust storms.The ecological environment of the study area was getting much worse then. While under the control of the water gate, there was a relatively slight lake-level fluctuation in the east lake, just about 1m.