Abstract: Improving the water use efficiency of vegetation can alleviate water shortage to a certain extent. To improve vegetation water use efficiency, in-depth-understanding of water-carbon cycle coupling in the ecosystem is needed. Time-varying gain model was coupled to improved photosynthesis-stomata conductance and dual source evapotranspiration models, to verify accuracy of the coupled models on daily basis, and to investigate water-carbon coupling, taking into account water utilization efficiency coefficient and stomatal conductance. Simulated evapotranspiration and total primary productivity were found to be higher when water-carbon coupling was not taken into account. Stomatal conductance showed a complete negative correlation, then remained stable with increasing stomatal conductance. Daily decoupling coefficient reached maximum in summer, annual average decoupling coefficient was found to be 0.1. At the group level at Beijing Forest Station, relative to the photosynthetic rate, transpiration rate is more sensitive to the decrease in stomatal conductance. Water and carbon coupling mechanism in the Beijing Forest Station area should prove helpful for ecosystem management and regulation.