Evapotranspiration is the important component of ecosystem hydrological and energy-balance. This study analyzed the evapotranspiration feature over flat underlying surface based on eddy covariance in Badaling Forest Farm in Beijing. We analyzed the evapotranspiration components via PT-Fi model and response to meteorological factors. The results of typical daily evapotranspiration variation in non-uniform underlying surfaces showing that the whole day were divided into 4 stages, 0:00 to 5:00 and 20:00 to 24:00 (night)was stable stage, 5:00 to 12:00 was increasing gradually stage, 12:00 to 14:00was fluctuant stage and 14:00 to 20:00 was decreasing stage. 24h averaged evapotranspiration in one month show that evapotranspiration monthly variation was accordance with the change of daily variation, expect the curve was smoother. According to the seasonal variation, evapotranspiration increased slowly from April to May, from June to September, the evapotranspiration arranged in the fastigium, while the evapotranspiration decreased rapidly from October. Total evapotranspiration during study period was 532.92mm, the components of ET--vegetation transpiration and soil evaporation were 394.48 mm and 138.44mm respectively, and the proportion were 74.02% and 25.98% respectively. The correlation between evapotranspiration and meteorological factors indicates that: correlation of solar radiation and evapotranspiration shows that: sunny day in summer (0.9484)>sunny day in spring (0.7909)>sunny day in autumn (0.6917), sunny day in summer (0.9484)>cloudy day in summer (0.8828). Correlation between evapotranspiration and temperature shows that: sunny day in summer (0.6735)>sunny day in autumn (0.4222)>sunny day in spring (0.1826)>, sunny day in summer (0.6835)>cloudy day in summer (0.647). Quantified the influence of meteorological factors on evapotranspiration, effect of solar radiation: sunny day in summer（0.3585 W﹒m-2/ W﹒m-2）>sunny day in spring（0.0474W﹒m-2/ W﹒m-2）>sunny day in autumn（0.0525W﹒m-2/ W﹒m-2），sunny day in summer（0.3585W﹒m-2/ W﹒m-2）>cloudy day in summer（0.2510W﹒m-2/ W﹒m-2）. Temperature：sunny day in summer（19.5535 W﹒m-2℃-1）>sunny day in spring（2.3224W﹒m-2℃-1）>sunny day in autumn （1.3516W﹒m-2℃-1），cloudy day in summer（21.0650W﹒m-2℃-1）>sunny day in summer（18.0420W﹒m-2℃-1）.The correlation analysis of evapotranspiration and rainfall shows that rainfall played negative effect on evapotranspiration, but there was a critical rainfall amount, that was to say the evapotranspiration would decrease until rainfall reached a certain amount. Analysis on combined effect of meteorological factors on evapotranspiration indicated that the maximum effect factor was temperature with the contribution rate of 40%~60% during research period. Contribution rate of solar radiation and relative humidity varied in different seasons.