Abstract: Reference evapotranspiration(ETo) is an important index for reflecting atmospheric evapotranspiration ability, evaluating climate drought, vegetation water-consumption. Based on the climatic data of 101 meteorological stations from 1961 to 2008 in Xinjiang，the annual ETo was calculated with Penman-Monteith equation recommended by FAO in 1998, and the spatial and temporal variation characteristic of the annual ETo and its climatic causes were analyzed by using linear regression method, Mann-Kendall abrupt test and three-dimensional quadratic trend surface simulation and inverse-distance square weighting residual error revising based on GIS. The main results show that the spatial distribution of annual ETo was obviously different in the different regions of Xinjiang. Generally, the annual ETo was larger in southern than it in northern, eastern was larger than western, basin (valley) was larger than in mountain regions in Xinjiang. By the comprehensive influence of the significantly increasing tendency of the mean temperature and air relative humidity, and the significantly decreasing of the sunshine hour and the wind speed, the annual ETo was significantly decreasing and it had significantly sudden decrease in 1981, but the decrease rate and range of the annual ETo had distinct regional differences. In general, the decrease rate and range of the annual ETo was large in the area of the ETo stronger, and vice versa. It has important significance of annual ETo decrease to reducing the amount of crop water requirement and farmland irrigation, reducing surface dryness, and improving vulnerable ecological environment in Xinjiang.