Using the data of monthly 20cm pan evaporation and its impact climatic factors(for instance, sunshine duration, wind speed, relative humidity, precipitation and daily temperature, et al.)at 14 meteorological stations in middle reaches of the Yarlung Zangbu River over Tibet from 1981 to 2005,the variations of annual and seasonal pan evaporation and the trend were analyzed by using the method of linear trend analysis. Main results were as follows:(1)Significant negative trend of annual pan evaporation were detected at the most stations, and the mean rate of decrease was 109.92 mm /10a (99% confidence level),especially in summer (-51.79 mm /10a, 99% confidence level). In terms of spatial distribution, the trend of annual pan evaporation increased in Milin county where the rate of increase was 41.22 mm /10a (95% confidence level),but the decreasing trends were found at the other stations,the rate of decrease was (1.23~348.68) mm/10a, which is obvious in Gongga county. (2)Significant negative trends of annual sunshine duration and wind speed were found in middle reaches of the Yarlung Zangbu River over Tibet. Significant positive trends of annual mean relative humidity and precipitation were detected. Significant increasing trend of annual mean temperature was detected, an increase of 0.33℃/10a. While the increasing trend of annual mean minimum temperature(0.52℃/10a)was higher than that of annual mean maximum temperature(0.27℃/10a), and the decreasing trend of daily temperature was significant(-0.29℃/10a).(3)It is thus obvious that the decrease of annual pan evaporation was mainly caused by the significant decrease of the annual sunshine duration and wind speed as well as obvious increase of annual mean relative humidity, the daily temperature and precipitation may also play an important role in an unknown mechanism.