Abstract: Excessive global CO2 emissions trigger a cascade of environmental issues including the greenhouse effect. The reverse water-gas shift (RWGS) reaction converts CO2 into syngas via hydrogenation, serving as a critical pathway for carbon resource utilization. Noble metal catalysts suffer from prohibitive costs, while transition metal catalysts readily produce methane as a byproduct. Pure zinc oxide exhibits low catalytic activity and severe sintering at high temperatures. Furthermore, the microscopic mechanism underlying the modulation of ZnO catalytic performance by Mg doping, as well as the quantitative correlation between doping content and catalytic activity, remain unclear. Against this backdrop, this work investigates Mg-Zn composite oxide catalysts to elucidate the doping modification mechanism, laying a theoretical and experimental foundation for developing high-efficiency catalysts for CO2 conversion. A series of composite oxides with varying Mg/Zn molar ratios were prepared by the coprecipitation method. Multiple microscopic characterization techniques, namely X-ray diffraction (XRD), CO2 temperature-programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS), were employed to analyze material microstructures. Catalytic performance was evaluated in an atmospheric-pressure fixed-bed reactor, and apparent activation energies were calculated using kinetic models. A 100 h long-term stability test was carried out on the optimal catalyst sample. Characterization results reveal that appropriate Mg2+ ions can be incorporated into the ZnO lattice to form homogeneous solid solutions without local elemental segregation. Mg doping induces the generation of oxygen vacancies, increases the electron density of Zn active sites, and creates abundant moderate-strength basic sites, thereby enhancing the adsorption and activation capacity of CO2. Catalytic performance measurements demonstrate that CO2 conversion rises initially and then plateaus with increasing Mg doping content. The Mg0.3Zn1O catalyst delivers optimal catalytic performance, with CO2 conversion rates reaching 11.2%, 20.2% and 31.5% at 500, 550 and 600 ℃, respectively—approximately three times that of pure ZnO. Its apparent activation energy is only 58.13 kJ/mol, substantially lower than the 144.31 kJ/mol measured for pure ZnO. During the 100 h continuous reaction test, CO2 conversion of the sample remains stable at around 30%, attributed to the solid-solution lattice that effectively suppresses grain sintering. Excessive Mg dilutes the active phase and leads to over-adsorption of CO2, which restricts further improvement in catalytic activity. Mg doping synergistically optimizes catalytic performance by regulating lattice defects, electronic structures, and surface basicity. The optimal Mg/Zn molar ratio is determined to be 0.3. This study clarifies the microscopic mechanism of doping modification and establishes the structure–activity relationship, providing comprehensive experimental support for the design of low-cost and highly stable ZnO-based catalytic materials.