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Evapotranspiration is one of the key components of hydrological processes. Assessing the impact of climate factors on evapotranspiration is helpful in understanding the impact of climate change on hydrological processes. In this paper, based on the daily meteorological data from 1960 to 2007 within and around the Aksu River Basin, reference evapotranspiration (RET) was estimated with the FAO Penman-Monteith method. The temporal and spatial variations of RET were analyzed by using ARCGIS and Mann-Kendall method. Multiple Regression Analysis was employed to attribute the effects of the variations of air temperature, solar radiation, relative humidity, vapour pressure and wind speed on RET. The results showed that average annual RET in the eastern plain area of the Aksu River Basin was about 1100 mm, which was nearly twice as much as that in the western mountainous area. The trend of annual RET had significant spatial variability. Annual RET was reduced significantly in the southeastern oasis area and southwestern plain area and increased slightly in the mountain areas. The amplitude of the change of RET reached the highest in summer, contributing most of the annual change of RET. Except in some high elevation areas where relative humidity predominated the change of the RET, the variations of wind velocity predominated the changes of RET almost throughout the basin. Taking Kuqa and Ulugqat stations as an example, the variations of wind velocity accounted for more than 50% of the changes of RET.