Abstract: The cement industry is one of the major sources of NO_x emissions in China, and NO_x control in cement kiln flue gas has become an important issue in the field of air pollution prevention and control. Selective non-catalytic reduction (SNCR), owing to its simple process and relatively low operating cost, has been widely applied in denitrification retrofits of cement kilns. However, the high concentration of CaO in cement kilns inhibits NO_x reduction and significantly affects the SNCR reaction process. To study the effect of CaO on the reduction of NO by NH₃, the reduction efficiency of CaO participating in NO reduction by NH₃ at different temperatures (700−1 600 ℃) and different oxygen concentrations (0−10%) was investigated in a high-temperature tubular furnace. Combined with the characterization and analysis of the reaction products, the influence mechanism of CaO on the reaction process between NH₃ and NO was revealed. The results show that CaO can significantly inhibit the process of NO reduction by NH₃ in the temperature range of 700−1 100 ℃. CaO surface will promote the oxidation of NH₂ and other groups by adsorbing NH₂ and other groups and active oxygen species, thus reducing the efficiency of NO reduction by NH₃. When the temperature exceeds 1 100 ℃, CaO is sintered at high temperature, and the specific surface area and average pore size decrease, which reduces the activity of CaO and significantly weakens its effect on the reduction of NO by NH₃. With the decrease of O₂ concentration, the catalytic effect of CaO changes from catalytic oxidation of NH₃ to catalytic decomposition of NH₃, which reduces the proportion of chemisorbed oxygen on the surface of CaO and weakens the oxidation of NH₃, so that the inhibitory effect of CaO on the reduction of NO by NH₃ gradually weakens. The results systematically elucidate the underlying mechanisms by which CaO affects the SNCR process from the perspectives of temperature and atmospheric conditions. They provide an important theoretical basis for the optimization and design of SNCR denitrification systems in cement kilns, and offer valuable guidance and engineering implications for the rational selection of the NH₃ injection temperature window, precise regulation of O₂ concentration within the kiln, and further improvement of NO_x reduction efficiency in cement kilns.