Abstract: Based on daily meteorological data form 1961 to 2009 over Midsteam of Yarlung Zangbo River in Tibetan Plateau, Using Penman -Monteith method, The temporal and spatial distribution characteristics of PE (potential evapotranspiration) was calculated and analyzed in this paper.And using Multiple Regression, The Contribution rate to PE variations by climate factors was also calculated quantitatively.The reaserch shows that:In the recent 49 years, The PE of LhaSa showed a trend of increasing obviously, The growing rate reached 8.21mm/10a, Shigatse and Gyantse has been decreased not obviously, but Tsedang decreased obviously, The reduction reached-24.71mm/10a.The variation trend of PE has a big seasonal difference, an abrupt time is in 1993.In the background of global warming, The significant reduction of average wind speed has explained the cause of reduction of PE from the climate point of view.