An open path eddy-covariance (EC) system and a microclimate monitor system were employed to continuously measure the carbon, water, and energy exchange between a poplar (Populus euramericana) plantation and the atmosphere during the growing season of 2007. The study site was located on dried flood plain dominated by sandy soils in Daxing District, southern Beijing, China. The objectives of our study were to a) qualify the dynamics of carbon, water fluxes and water-use efficiency (WUE) at an ecosystem level under various soil water conditions and b) examine the effects of environmental factors influencing WUE under different soil moisture conditions. We found that there was a significant relationship between gross ecosystem productivity (GEP) and evapotranspiratoin (ET) under different soil water. When relative extractable soil water (REW) was less than 0.1, both GEP and ET maintained at a low level and did not changed with the variation of environmental factors. GEP and ET increased as soil water contents (VWC) increase, but the greater effect of VWC on ET over GEP resulted in negative correlation between WUE and VWC when 0.1<REW<0.4. When REW was higher than 0.4, climate factors showed a significant influence on the variation of GEP and ET through stomata. Air temperature (T_a) and photosynthetically active radiation (PAR) were the main factors for the variation of GEP while air temperature (T_a) and saturated vapor pressure deficit (VPD) showed a significant effect on changes of ET. WUE decreased with the increasing VPD due to the higher sensitivity of ET to VPD than GEP. VWC was between 5.2% and 8.8% (0.1<REW<0.4) in this fast-growing poplar plantation, vegetation could utilize soil water efficiently and reached high rates of water use efficiency and carbon sequestration. We concluded that soil water condition was a key factor influencing WUE of the poplar plantation. Assessment of forest carbon sequestration and water consumption under a climate change environment must consider future precipitation regime in the study region.