The increasing carbon dioxide emissions have a huge impact on the global environment. Carbonation reaction of CaO is regarded as a potential method to capture carbon dioxide. The density functional theory calculations have been performed to investigate the adsorption of CO2 on CaO(100) surface. This paper systematically studied the adsorption of CO2 at different adsorption sites on CaO(100) surface and the influence of adsorption angle on adsorption energy. Based on the studying of adsorption sites, adsorption energy and electronic structure of the CO2/CaO(100) systems, chemical adsorption mainly happens when CO2 molecules are absorbed on the CaO(100) surfaces, but physical adsorption may also happen. The research found that CO2 molecules reacted with surface O atom through C, forming monodentate surface carbonate species and tridentate carbonate. Among them, low-coordinated monodentate ligands have a higher stability than tridentate ligands due to the shorter C–O_S bond length of monodentate ligands.