Abstract: The interaction mechanism between the unburned carbon in fly ash and the arsenic pollutants in flue gas such as As, AsO, AsO₂ and As₂O₃ was studied based on the density functional theory. The results show that the elemental arsenic is preferentially adsorbed at the carbon bridge site, with an adsorption energy in the range (-5.95)-(-5.88) eV; the AsO molecule preferentially combines with the unburned carbon in a way that the arsenic and oxygen atoms are bound with the surface carbon atoms respectively, forming a most stable configuration with an adsorption energy of -7.87 eV. When AsO₂ is dissociated on the unburned carbon surface and form an AsO molecule and a surface reactive oxygen species, the system is the most stable, possessing an adsorption energy of -10.65 eV. While once the two oxygen atoms in a trigonal bipyramid As₂O₃ molecule first collide with the unburned carbon surface, it will be dissociated to small molecules of AsO and AsO₂, forming a covalent bond with surface carbon. The adsorption energy is significantly reduced to -10.64 eV, compared with the undissociated case. The unburned carbon in fly ash is easy to bind with AsO or AsO₂ small molecules, which locally tends to form a special five-member ring structure. Compared with As, AsO and AsO₂, the most toxic trivalent arsenic As₂O₃ is chemically stable and not easy to adsorb. Catalytic pyrolysis of As₂O₃ into small molecules of AsO and AsO₂ is expected to be a feasible measure to control the arsenic pollution in coal-fired power plants flue gas.