Abstract: The influence of H₂O on the adsorption of SO₂ on CaO (001) surface was investigated by density functional theory (DFT). The results indicate that H₂O can have an effect on the adsorption geometries for SO₂ on the CaO (001) surface. When SO₂ is adsorbed to the CaO surface near a water group of different forms (viz., -H₂O, -H, -OH and -H & -OH), the H group makes the adsorption energy 90 kJ/mol higher with the sulfur p-orbital shifting downward, whereas other groups have little effect on the adsorption energy. When SO₂ is adsorbs to the -OH surface and -H & -OH surface, bisulfite-like structures are formed, with lower adsorption energies and tending to form more stable structures as intermediates. When SO₂ adsorbs to the -H₂O surface, bisulfite-like structure is formed and the H₂O group decomposes to Ca(OH)₂ like-structure on the CaO surface; the new H groups mainly bond the bisulfite and make adsorption energy 45 kJ/mol higher.