An eddy covariance technique was used to measure the gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) during the 2011 - 2014 period over a moso bamboo forest at a site in Anji (AJ), China. WUE declined during the severe summer drought of 2013 when the vapor pressure deficit (VPD) was above 15 hPa, and was significantly higher than the average value. At AJ the average annual GPP, ET, and WUE were 1522 ± 73 C m^-2 year^-1, 693 ± 41 kg H₂O m^-2 year^-1, and 2.21 ± 0.23 g C kg^-1 H₂O, respectively. GPP and ET were closely correlated at AJ, with R2 equal to 0.64. The monthly GPP and ET showed strong positive linear, exponential or quadratic polynomial correlations to meteorological variables, including air temperature (Ta), net radiation (Rn), and VPD. WUE was negatively correlated to VPD, with 36.3% of the variation in WUE explained by VPD. This study contributes to the understanding of the carbon and water cycle response mechanisms in forest ecosystems in the climate change context and is significant in relation to forest carbon sequestration management.