As awater tower of Asia,the Tibetan Plateau has a large effect on East Asian and even global water cycle. The degrading alpine wetland over the plateau,working as an assurance of plateau ecological diversity, is an important origination of water and surface runoff,where land-air water exchange not only reflects the regional climate change,but also has great significance for ecological environment protection. The one-year data observed with 10-minute interval at Longbaotan wetland in the source area of the Yangtze River was analyzed by the method of the FAO Penman-Monteith to look for the evapotranspiration change over the degraded alpine wetland and its relationship with environmental factors. The results showed that 1) during the grass growth season, the potential evaporation had distinct diurnal and monthly changes; the actual evapotranspiration was low in winter and high in summer with a peak in summer; 2) during the observation period,there was heavy water loss due to evapotranspiration exceeding precipitation; 3) soil temperature exerted an impact on the evaporation process; the soil temperature at 5-cm depth correlated more significantly with evaporation; the increase in soil moisture could provide plenty of moisture for evaporation process; 4) during a year,temperature is the key factor influencing the actual evapotranspiration; but in a sunny day during grass growth period,the evapotranspiration and radiation varies in the almost same pattern,and surface air temperature has less influence on the evapotranspiration, which correlates negatively with the relative humidity near surface.