It is of great importance to project the response of carbon and water fluxes of terrestrial ecosystems with climate change and to develop science-based biological climate change mitigation strategies. We used our continuously measured long-term carbon and water flux data for a poplar plantation (Populus euramericana CV. "74/76") to calibrate and validate a widely applied Biome-BGC model to accurately simulate gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) and to project their responses to climate change. The climate change scenarios were designed with different levels of rising temperature (T), precipitation change (P), and atmospheric CO₂ concentration (C). Results showed that the Nash-Sutcliffe coefficient (NS) of the simulated GPP and ET were 0.69 and 0.63, respectively, with a root mean square error (RMSE) of 1.94 g C m^-2·^-1 and 0.88 mm/d, respectively, which indicated that the calibrated Biome-BGC model could be effectively used for modeling their responses to climate change. Under future climate change scenarios, the overall responses of GPP and ET were influenced by a combined effect of C, T, and P. In addition, the individual responses of GPP and ET to these climatic factors varied. Rising temperature and decreasing precipitation caused a decrease in GPP, while an increase in precipitation and atmospheric CO₂ concentration resulted in an increase in GPP. The enhancement of GPP with increasing atmospheric CO₂ concentration was 28%-44%, which was much higher than that of rising temperature (1%-5%) and precipitation (3%-10%). However, the variations in ET only responded to a precipitation change of 5%-14%. As a result, WUE (GPP/ET) decreased with rising temperature and an increase in precipitation, while increased with a decrease in precipitation and rising atmospheric CO₂ concentration. The rising atmospheric CO₂ concentration enhanced WUE by 27.7%-43.6%, which was much higher than that effect of rising temperature (1.2%-5.8%) and precipitation (1.2%-3.5%). Compared with the current WUE (2.8 g C/kg H₂O), the largest increase and decrease in WUE would occur under scenarios C2T2P1 and C0T3P0, which are 45.4% and 5.8%, respectively.