CO₂ and water are important substrates for plant photosynthesis. Changes in CO₂ concentration or soil water will lead to corresponding changes in the photosynthetic characteristics of plant. Maize (Zea mays L.) is an important C₄-plant and has become the first cultivated crop of China. Maize yield in Northeast China is accounting for about 1/3 of the national total maize yield, and having a central role in ensuring the food security of county. However, the combined effects of CO₂ concentration and water status on photosynthesis efficiency, water use efficiency and yield of maize in Northeast China are not well understood. Maize variety Danyu39 was grown in the open top chambers(OTCs)to study the effects of contrasting CO₂ concentrations(390, 450 μmol/mol and 550 μmol/mol)and increasing precipitation(0 and +15%, which based on the 6、7、8 month average monthly precipitation, i.e. 88.7, 153.9 mm and 139.8 mm during 1981-2010 in Jinzhou)on photosynthetic characteristics and crop yield. By using LI-6400 portable photosynthesis system, the photosynthetic parameters data were measured and the modified rectangular hyperbolic model was used to fit the light response curves of 6 treatments(C₅₅₀W_+15%, C₅₅₀W₀, C₄₅₀W_+15%, C₄₅₀W₀, C₃₉₀W_+15% and C₃₉₀W₀). The results showed that maize leaf net photosynthetic rate(P_n)increased under the conditions of increasing CO₂ concentration and irrigation, respectively. Moreover, the irrigation effect was greater than elevated CO₂ concentration. With transpiration rate(T_r)decreasing, water use efficiency(WUE)increased greatly. Stomatal conductance(G_s)showed decreased trend with the increase of CO₂ concentration, but G_s increased with irrigation, and the effect of irrigation was less than elevated CO₂. Although the intercellular CO₂ concentration(C_i)increased significantly by the influence of elevated CO₂ concentration, but there were no significant effects by irrigation. Light response parameters were obviously different under the combined impact of increased CO₂ and irrigation. As the increase of CO₂ concentration, the maximum net photosynthetic rate(P_nmax)and light saturation points(LSP)increased gradually, and human irrigation was superior to the natural precipitation treatments. With the increase of CO₂, the difference of the light compensation points(LCP), the quantum efficiency of the light compensation points(φ_c)and dark respiration rate(R_d)became smaller between the irrigation and natural precipitation treatments. The leaf area increased by 11.56%, 3.31% and 0.45% for irrigation treatments compared with natural precipitation treatments under 390, 450 μmol/mol and 550 μmol/mol CO₂ concentrations, respectively, and dry matter accumulation increased by 14.69%, 8.09% and 1.01% respectively, eventually the yield increased by 10.47%, 12.07% and 8.96% respectively. So the increase of photosynthetic capacity resulting in the increase of maize grain yield under elevated CO₂ concentration and adequate irrigation. The results from this study can be of great help in evaluating the possible consequences of climate change on crop photosynthetic capacity and yield in Northeast China, and are crucial to help decision-makers to adjust measures for adaption to climate change.