Abstract: The availability of CO₂, a substrate for photosynthesis, affects the photosynthesis process and photosynthate production. Using the Li-6400-40B, we measured the photosynthetic electron transport rate and the photosynthetic light-response curves of soybean (Glycine max) leaves at different CO₂ concentrations (300, 400, 500 and 600 μmol·mol^-1). By fitting these parameters with a mechanistic model characterizing the light response of photosynthesis, we obtained aseries of photosynthetic parameters, eco-physiological parameters, as well as the physical parameters of photosynthetic pigments. The results showed that the electronic use efficiency, maximum electron transport rate, and maximum net photosynthetic rate increased with the increase of CO₂ concentration. The light compensation point and dark respiration rate decreased with the increase of CO₂ concentration. In addition, the light-use efficiency and intrinsic (instantaneous) water-use efficiency increased with the increase of CO₂ concentration, and their values differed significantly among different CO₂ concentrations. There was no significant difference on the maximum carboxylation efficiency among different CO₂ concentrations. Those results suggested that CO₂ concentration could affect the primary light reaction of photosynthesis in soybean leaves, and thus higher CO₂ concentration could decrease the minimum average lifespan of excitons at the lowest excited state, which would enhance the velocity of light energy transport and the use efficiency of photosynthetic electron flow.