Energy, water and CO2 flux at the soil-atmosphere interface is a key interest among ecosystem researchers. The Simultaneous Heat and Water (SHAW) Model describes radiation energy balance, heat transfer and water movement within the Soil-Plant-Atmosphere Continuum, but has no provisions for CO2 flux. This study coupled the components of solar radiation and water transfer within a plant canopy in SHAW with a biochemical photosynthesis model. The SHAW model provides leaf water potential to the photosynthesis model to calculate intercellular CO2 (Ci) through stomatal control in each layer within the canopy, and then provides solar radiation, air temperature and humidity used to calculate photosynthetic rate (Pn) within each canopy layer. Stomatal conductance (gs) was calculated by a revised Ball-Berry model, describing the relationship between gs and Pn, which was a feedback from the photosynthesis model to SHAW to calculate energy and water transfer, and in turn the leaf water potential. The photosynthesis model was run iteratively with the SHAW leaf energy balance within each canopy layer to reach convergence in leaf temperature. Because leaf water potential comprehensively reflects the effect of solar radiation, temperature, and humidity, which is the most fundamental element influencing stomatal opening, SHAW takes leaf potential as the sole factor controlling stomatal conductance. After including the relationship between stomatal conductance and photosynthetic rate, computed stomatal conductance was able to decrease in response to CO2 increase. The photosynthesis model was validated, and photosynthesis, transpiration, stomatal conductance, and Ci at a leaf show appropriate response to changes in light and CO2. The extended SHAW (SHAW-Pn) model was capable of simulating CO2 flux over plant canopies. It performs excellently in simulating net radiation, sensible and latent heat, and CO2 fluxes over winter wheat field in the North China Plain (36„a57„SN, 116„a36„SE, 28 m above sea level). The root mean square difference (RMSE) of simulation for net radiation, latent and sensible heat fluxes is 38.4, 46.0, and 26.3 W m-2, respectively. The RMSE of CO2 flux simulation is 0.16 mg m-1 s-1.