Mercury ought to be removed to near zero in O-2/CO2 due to safety issues. The mercury adsorption species can influence the efficiency of the mercury adsorbent. The mercury adsorption species on the activated carbon determined the mercury adsorption efficiency by the activated carbon. To elucidate the mechanism of the mercury adsorption with SO2 under the O-2/N-2 and O-2/CO2 atmosphere, mercury adsorption species on the samples of adsorbed activated carbon (AC-Hg) were investigated by the mercury temperature-programmed desorption (TPD) method, and the mercury desorption activation energy was also calculated. It could be found that Hg-OH, HgS, HgO (yellow), Hg O and HgSO4 were the major forms under O-2/CO2 atmosphere. Furthermore, it also indicated that the chemical adsorption with strong Hg O bond and high mercury desorption activation energy was the dominant form under O-2/CO2 atmosphere. The results of the present work implied that temperature-programmed desorption could identify mercury species on the AC-Hg clearly. The sorbate-sorbate bond was stronger than the sorbate surface bond.