Abstract: CaO in the cement kiln inhibits NOx reduction, which 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 (700-1600 °C) 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 800-1000 °C. 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 1100 °C, 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.