The adsorption mechanism of As2O3 on carbonaceous surface modified with oxygen functional complexes was studied using density functional theory to understand the effect of oxygen functional complexes on arsenic adsorption. Full-parameter geometrical optimization and single point energy were calculated on B3LYP/def2SVP and B3LYP/def2-TZVP level. Results showed that As2O3 adsorption on bare carbonaceous surface took place in physical as well as chemical way. The adsorption energies were between -2.07 kJ/mol to 480.20 kJ/mol. Compared to armchair model, zigzag model was more suitable as a carbonaceous sorbent. The participation of oxygen functional complexes greatly promoted the surface activity of carbonaceous surface and its adsorption capacity on arsenic. The adsorption energies of arsenic on carbonaceous surface with oxygen functional complexes were between -111.56 kJ/mol to 669.46 kJ/mol. The promotion order of oxygen functional complexes on surface activity was: phenol > lactone > carbonyl > semiquinone > carboxyl. Oxygen functional complexes promoted adsorption capacity of carbonaceous surface through enhancing the activities of neighboring carbon atoms rather than directly providing active sites. Mayer bond order was a reliable way to understand the adsorption process of arsenic on carbonaceous surface. This study provides a new idea for using modified carbonaceous sorbent to remove arsenic pollution from power stations.