The relationship between cloud cover variation and atmospheric circulation is analyzed with the JanuarytoDecember mean cloud cover data of four stations from 1961 to 2000 over the Qilian Mountain area by composite, correlation and spectral analysis methods. The results show that the cloud cover over the Qilian Mountain area is affected by the Western Pacific subtropical high, QinghaiX izang Plateau monsoon, zonal circulation, and solar activities. When the subtropical high area enlarges, moves northwards and intensifies the zonal circulation in midlatitudes, it thus induces the subtropical front to move northwards, decreases the times of the cold and warm air meeting over the Qilian Mountain area, and eventually causes the decreased cloud cover; and vice versa. The stronger (weaker) the plateau monsoon, the more (little) cloud cover over the Qilian Mountain.