In this study, we used an opentop chamber method to control the environmental CO₂ concentration. Environmental CO₂ concentration (Ca, 390 μmol·mol^-1) and elevated CO₂ concentration (Ce, 700 μmol·mol^-1) were set from 2018 to 2019. The effects of CO₂ concentration increase on the activities of key enzymes of photosynthetic carbon assimilation, net photosynthetic rate and grain yield of functional leaves of maize and peanut were investigated, and the mechanism of the effects of CO₂ concentration increase on the photosynthesis of maize and peanut intercropping system was clarified. It provides a theoretical basis for high yield and high efficiency of maize and peanut intercropping in the future CO₂ concentration increasing environment. The results indicate that: (1) compared with Ca, the activities of PEPC, PPDK, NADPMDH, Rubisco, GAPDH and Ru5PK in functional leaves of intercropping maize under the condition of Ce were increased. In particular, PEPC, NADPMDH, and PPDK at 43 days after seedling, Rubisco, GAPDH and Ru5PK 59 days after seedling reached a significant level of difference. Under the condition of Ce, phosphorus application has a positive regulatory effect on it. (2) The activities of Rubisco, GAPDH, Ru5PK and FBPase in functional leaves of intercropping peanuts increased by Ce. At 43 days after seedling and 59 days after seedling, the difference reached a significant level. Phosphorus application significantly improved the activities of Rubisco and FBPase of intercropping peanut under the condition of Ce. (3) The net photosynthetic rate of intercropping maize and intercropping peanut increased significantly under the treatment of Ce. The yield of intercropping maize, intercropping peanut and intercropping system increased by 4.4%-52.0%, 10.3%-24.0%, and 5.7%-47.0%, respectively, all reached significant different level. It indicted that Ce could enhance the grain yield of maize and peanut, the key lies in the improvement of enzyme activities of PEPC, PPDK, Rubisco, GAPDH, Ru5PK and Ru5PK of intercropping maize, as well as the Rubisco, GAPDH, Ru5PK and FBPase of intercropping peanut, to strengthen the carboxylation of CO₂ fixation ability, thus improve the photosynthetic rate of intercropping maize and intercropping peanut. Phosphorus application has a positive regulatory effect on them.