Abstract: Based on the the data of cloud amount during 1961-2010 form Tongde meteorological station in the Three-River Source region, the variation of the low cloud amount was analyzed, and also the relationship between cloud amount and interrelated climatic factors was discussed. The results showed that the mean amount of low cloud increased significantly since 1961,four seasons mean amount of low cloud showed an increasing trend,spring,summer and winter mean amount of low cloud increased significantly. Average low cloud cover is bimodal distribution, June and July are the low cloud cover in most months of the year, December was the lowest value. Mutation analysis indicates that the mean low cloud of year and spring took place in 1964,1965 by most to least, and from least to most in 2002,2001;the mean low cloud in autumn abrupt change took place in 1963 from most to least; the winter abrupt change took place in 2000 form least to most; the summer was not mutated. the temperature and water vapor pressure factor is being affect most to low cloud amount; followed by hours of sunshine ;the third is average wind speed.