Drought decreased the optimal temperature for photosynthesis and significantly reduced carbon fixation of Moso bamboo forest. VPD is a superior indicator for simulating the response of GPP to drought. The severity and frequency of drought in the subtropics and tropical marginal convective zones are increasing and consequently greater attention is being given to the effects of drought on the carbon balance of forest ecosystems. Moso bamboo (Phyllostachys edulis (Carriere) J.Houz.) is an important economic forest type in China that is vulnerable to drought. Using an eddy covariance technique and model simulation, this study analyzed the responses of the carbon fluxes of Moso bamboo to environmental factors during a drought. It was found that temperature had a restricting effect on photosynthesis, and that the optimal temperature for photosynthesis was left-shifted, indicating that drought decreased the optimal temperature for photosynthesis and changed the response of gross primary productivity (GPP) to temperature. In comparison with other environmental factors, the vapor pressure deficit (VPD) was established as a much better indicator for simulating the response of GPP to drought. By including the VPD limitation of GPP in the eddy covariance light use efficiency model, it was found that the model could explain the response of GPP to drought well, reducing the uncertainty in GPP (e.g., the overestimation of GPP under drought-stress conditions). A dynamic linear regression showed drought decreased carbon fixation by roughly 12.7 % of GPP and 44.8 % of the net ecosystem CO2 exchange, but it had no obvious influence on ecosystem respiration.