Abstract: Global CO₂ concentration rapidly increases over the past few centuries, particularly in recent decades, and the greenhouse effect is sharply accelerated by the increase of CO₂ emissions. CO₂ emissions are mainly produced from fossil fuel combustion, in particular coal-fired power plants. Calcium looping, i.e. the carbonation/calcination cycles of CaO, is one of the most promising CO₂ capture technologies for coal-fired power plant. At the same time, coal-fired power plants are also one of the main sources of NO_x emissions. CO can be used as a reducing agent for NO due to its low cost and low toxicity. CaO is a suitable catalyst for NO reduction by CO. The combustion of biomass is used to provide the required energy for the decomposition of CaCO₃ in the calciner. Few unburned char and CaO flow into the carbonator, where the unburned char reacts with O₂ in the flue gas to generate CO. Thus, the simultaneous NO removal and CO₂ capture in the carbonator of the calcium looping process can be realized. However, the reaction mechanism of the effect of CaO on NO removal by CO in the carbonation stage is unclear. The mechanism of NO reduction with CO catalyzed by CaO in the carbonation stage is investigated by the density functional theory (DFT) calculations. The structural, adsorption, and reaction path parameters, including atomic layout, bond length, adsorption energy, and energy barrier of CO and NO molecules on the CaO (100) surface, are determined. The optimal adsorption sites of CO and NO molecules on the CaO (100) surface are O-top sites, and the adsorption energies are −0.35 and −0.79 eV, respectively. The CO and NO adsorption energies on the CaO (100) surface in the presence of CO₂ are −0.36 and −0.20 eV, respectively. The co-adsorption of CO and NO molecules appears feasible on the CaO (100) surface, while CO₂ exhibits obvious inhibition for the co-adsorption. The reaction path of NO reduction by CO on the CaO (100) surface undergoes two elementary reaction stages: CO₂ formation and adsorption, and N₂ generation. Finally, one N₂ molecule and three \rmCO_3^2- ions are formed, with a total energy barrier of 11.08 eV. CO₂ formation and adsorption (R→IM1→IM2) are the rate-determining steps of the whole reaction process.