Eddy-Covariance（EC）open path system and microclimate monitor system were employed in this study to investigate energy allocation and diurnal variation of evapotranspiration (ET) in growing season of 2005. The study was located in DaXing District, Beijing on a sandy soil in an 11-year-old poplar plantation.The primary objective was to compare the energy fluxes, the relation of diurnal change of ET, and microclimate at ecosystem level before and post a precipitation event. Field data suggested that the diurnal variation of ET showed a single-peak curve under serious water stress with an average daily ET of 2.4 mm, while a multi-peaks curve with a daily average of 4.5 mm post the 56.89 mm precipitation on July 23, 2005. Our analysis of energy balance showed that the contribution of latent and sensible heats to the available energy (Rn-G) varied greatly before and after the rain event. Prior to the precipitation, LE/Rn ratio was lower than that of post rain event, averaging 54.88% and 83.80%, respectively. This suggested that the most of energy was used for ET when soil water was sufficient in this ecosystem. Under water stress, net radiation (Rn) and soil moisture played important roles in ecosystem ET, while other environmental factors showed minor influence on ET. We also found high correlations between soil conditions, microclimate, and Rn with ET under moist soil condition. Air temperature had little effect on ET regardless of soil moisture condition. The correlation of soil moisture and ET was low prior to precipitation, suggesting a minor role in affecting ecosystem ET.