Abstract: Based on the daily data of 112 meteorological stations from 1960 to 2009,the potential evaporation(PE) is calculated using the UN Food and Agriculture Organization(FAO)Penman-Monteith model,and then the humid index is reconstructed from the PE in Northwest China.With the method of Inverse Distance Weighted,Mann-Ken-dall,etc,the temporal and spatial variations of humid index are discussed,as well as its impact factors.The result indicates that the humid index increases in the last 50 years with an increasing rate of 0.006/10a,which are obvious in spring and winter.From northwest to southeast,the humid index displays a marked high-low-high trend in spatial distribution.According to the spatial variation,the study areas can be divided into three sub-regions:obvious increasing,light increasing and continuous decreasing.The influential factors have different impacts on the surface humidity index during different times in the past 50 years,showing a positive correlation with precipitation and-relative humidity,and negative correlation with sunshine duration,potential evaporation and wind speed.