Mercury and selenium are two typical heavy metal pollutants in coal combustion flue gas. The adsorption mechanism of Se-0 and SeO2 over an activated carbon (AC) adsorbent and the specific role of selenium species in Hg-0 elimination were systematically studied by density functional theory calculation. The adsorption complex structures of a single Se(0 )atom and SeO2 molecule on the AC surface were optimized. The adsorption energy in a range of -238.97 to -545.80 kJ/mol indicated that chemisorption was the main adsorption mechanism. The adsorption properties of multiple Se-0 atoms and SeO2 molecules on the AC surface were studied, and the most possible adsorption pathway was given. Multi-Se-0 atoms preferred to adsorb on neighboring active sites of the AC surface to share their atomic orbitals. Instead, multi-SeO2 molecules preferred to occupy the active sites of the AC surface without any interaction. The multicomponent adsorption processes of He and selenium species on the AC surface were studied by calculating adsorption energies, electron charge transfer, and atomic bond populations. For the coadsorption of He and Se-0, they preferred to adsorb on different carbon atoms in a separate way. In contrast, SeO2 tends to decompose into Se and SeO ions, and He can achieve stable configuration through interacting with the SeO ion and AC surface with an adsorption energy of -354.92 kJ/mol. This study indicated future potential for applying activated carbon in multicomponent adsorption of selenium and mercury in coal combustion flue gases.