Based on the daily data of 24 meteorological stations  from 1960 to 2006 and combined with the Penman-Monteith model, the changing trend of potential evaporation in the Tianshan Mountains was analyzed in this study. By the method of IDW under ArcGIS the spatial distribution of potential evaporation was drawn in order to research regional difference. And the multivariate regression method was used to discuss the dominant factor influencing potential evaporation. The results indicated that since the 1960s the annual potential evaporation has presented an undulated decreasing trend, and the decreasing trend was more evident since 1986.  It began to increase since 2000. There was a decreasing trend in general because the changing rate of annual potential evaporation varied at -2.48 mm/a. The potential evaporation increased in autumn, however, decreased in other seasons especially in spring. The wind speed was a dominant factor influencing potential evaporation, while temperature was the key factor in autumn.