Evaporation is an important factor in water balance of lakes. The evaporation pan is a traditional method to observe water evaporation, while it cannot represent actual water evaporation. So the characters of actual water evaporation are still unclear. In this study, the actual water evaporation was measured by an eddy covariance system over the Lake Poyang in August of 2013. We analyze the diurnal and monthly variations of water evaporation and its main influencing factors, and compare it with pan evaporation measured at Tangyin station. The results show that the water evaporation has strong turbulence in diurnal variations in the range of 0-0.4mm/h. The diurnal variation of water evaporation is mainly controlled by the wind speed. The daily water evaporation has similar trend with daily pan evaporation in August over the Lake Poyang. The average water evaporation in August is 5.90mm/d, which is higher than 4.6% for average pan evaporation(5.65mm/d). The ratio of water evaporation to pan evaporation in August shows decreasing trend, and the ten-day average ration was 1.24, 1.00, 0.92, respectively. The daily water evaporation in August has good relationship with wind speed and relative humidity, and pan evaporation was significantly related to net radiation, air temperature, the saturation vapor pressure deficit and relative humidity. This is due to evaporation pan has small volume and weak water heat storage capacity comparing with the lake, thus it easily affected by environmental factors.