Based on the data of cloud amount during 1951－2007 from seven weather stations in Tianshui，the variation of the total cloud amount and low cloud amount was analyzed，and also the relationship between cloud amount and interrelated climatic factors was discussed．The results showed that the mean total amount of cloud varied little and mean amount of low cloud increased by 0．25%every ten years since 1951．The rainy days with different rainfall level correlated well with cloud amount．It also indicated that the precipitation varied with cloud amount，the annual precipitation increased by 156 mm when annual mean cloud amount increased by 10%．
When seasonal mean cloud amount increased 10%，the precipitation increased by 25 mm，75 mm，35 mm and 3．8 mm in spring，summer，autumn and winter，respectively．When the cloud amount increased by 10%，the mean temperature in spring and summer decreased by 0．6℃and 0．5℃，but there was no correlation between cloud amount and mean temperature in winter and autumn．The sunshine hours was seriously influenced by cloud amount，when cloud amount increased by 10%，the sunshine hours decreased by 102．2 h in spring，90．8 h in summer，87．7 h in autumn and 65．3 h in winter．There was positive correlative between mean cloud amount and relative humidity，when cloud amount increased by 10%，the relative humidity increased by 4%in summer，3%in spring，autumn and winter．The days with＞80%cloud amount reduced year by year in spring．