CaO-Based materials are considered to be feasible sorbents for the removal of both HCl and CO₂. Theoretical calculations based on density functional theory (DFT) were performed to investigate the micro-mechanism of HCl/CO₂ capture in a CaO-based chemical looping process. The results show that HCl preferentially interacts with surface O atoms on the CaO (100) surface, with a highest adsorption energy of −1.85 eV. The hydroxyl group, OH⁻, is observed during the adsorption process. At rising coverage, increased repulsion between the HCl molecules results in a lowering of the adsorption energy per HCl. The binding mechanisms of HCl and CO₂ to CaO involve chemisorption and there is competing adsorption behavior between them due to positive interaction energy between coexisting HCl and CO₂. The absolute values of the absorption energy and reaction barrier for CO₂ adsorption and desorption in the HCl/CaO system are much lower than those on the pristine CaO (100) surface, which indicates the inhibition of CO₂ adsorption and the promotion of CO₂ desorption in the presence of HCl. With HCl adsorbed, the peaks for the p orbitals of the ortho-O and para-O atoms decrease by 13% and 5% in electron density, respectively, near to the Fermi level, and the ortho peak shifts from −1.79 to −0.45 eV; these aspects are inferred to play important roles in CO₂ adsorption/desorption on CaO.