Cu/Mg-MOF-74 has several advantages, such as high specific surface area, adjustable microporous structure, alkali metal active site, excellent CO₂ adsorption, and good photocatalytic activity. However, how the molar ratio of Cu/Mg (Cu/Mg ratio) affects its CO₂ adsorption selectivity in a simulated flue gas is still unclear. Here, a synthesized Cu/Mg-MOF-74, with series of Cu/Mg ratios, using the solvothermal method was analyzed about its CO₂ photocatalytic performance, CO₂ and N₂ uptake, and pore structure. The CO₂ adsorption selectivity was calculated to reveal the effect of Cu/Mg ratio on CO₂ and N₂ uptake and selectivity. The results indicate that the photocatalytic activity of Cu/Mg-MOF-74 for CO₂ reduction to CO and H₂ initially increases and then decreases with Cu/Mg ratio decreasing. At the Cu/Mg ratio of 0.6/0.4, the yield of CO and H₂ by photocatalytic reduction is the highest, showing up to 10.65 and 5.41 μmol·h^-1·g_cat^-1 (1 MPa, 150 ℃), respectively. Furthermore, CO₂ and N₂ uptakes of Cu/Mg-MOF-74 increase as the Cu/Mg ratio decreases, and the increase in CO₂ uptake is more pronounced. At the Cu/Mg ratio of 0.1/0.9, the CO₂ and N₂ uptakes are the largest, reaching 9.21 and 1.49 mmol·g^-1 (273.15 K, 100 kPa), respectively. Their area and volume of micropore (d₁ ≥ 0.7 nm) and ultramicropore (d₂ < 0.7 nm) increase as the Cu/Mg ratio decreases. At the Cu/Mg ratio of 0.22/0.78, the area and volume of micropores and ultramicropores are larger than those of Mg-MOF-74. The selectivity of Cu/Mg-MOF-74 increases correspondingly with Cu/Mg ratio decreasing and CO₂ concentration increasing. CO₂ adsorption on Cu/Mg-MOF-74 is a combination process of pore-filling and Mg²⁺ chemical adsorption in which the micropore volume is the key factor affecting its adsorption performance. All above data demonstrate that modulating the Cu/Mg ratio can promisingly regulate the pore structure of Cu/Mg-MOF-74, CO₂ uptake, and selectivity.