The　adsorption　of　mercury　and　mercury　chloride　on　a　CaO(001)　surface　were　investigated　by　the　density　functional　theory　(DFT)　by　using　Ca9O9　cluster　embedded　in　an　electrostatic　field　represented　by　178　point　charges　at　the　crystal　CaO　lattice　positions.　For　the　mercury　molecular　axis　normal　to　the　surface,　the　mercury　can　only　coordinate　to　the　O2-　anion　and　has　a　very　weak　binding　energy　of　19.649　kJ/mol.　When　the　mercury　chloride　molecular　axis　is　vertical　to　the　surface,　the　Cl　atom　coordinate　to　the　Ca2+　cation　and　has　binding　energy　of　23.699　kJ/mol.　When　the　mercury　chloride　molecular　axis　is　parallel　to　the　surface,　the　Hg　atom　coordinate　to　the　O2-　anion　and　has　binding　energy　of　87.829　kJ/mol,　which　means　that　the　parallel　geometry　is　more　stable　than　the　vertical　geometry.　The　present　calculations　show　that　CaO　injection　could　substantially　reducing　gaseous　mercury　chloride,　but　have　no　apparent　effect　on　the　mercury,　which　is　compatible　with　the　available　experimental　results.　The　research　method　will　provide　the　valuable　information　for　the　optimizing　and　selecting　sorbent　of　the　trace　element　in　flue　gas.