In this study, hydrogen, hydroxyl, carbonyl, carboxyl, amine, epoxy functional groups are introduced into various pore structures of activated carbon, to investigate their effect on the adsorption of 4-chlorophenol by molecular simulation. Adsorption isotherm of 4-chlorophenol is then calculated by sing Grand Canonical Monte Carlo (GCMC) simulation. The simulation results showed that, on the none structure, the 4-chlorophenol adsorption capacity reached the maximum value with a pore size of 1.5 nm. In contrast, when a functional group is present on the surface, activated carbon with a 4 nm pore is more dominant in terms of adsorption capacity. All functional groups exert a great influence on the threshold pressure and reduce the adsorption capacity at high pressure. But at medium and low pressures, the adsorption capacity of activated carbon increases rapidly in the presence of functional groups. The simulation results also showed that the carboxyl functional group gives superior adsorption capacity at medium and low pressures. The ability of activated carbon to form adsorption bonds with 4-chlorophenol was further investigated by comparing the adsorption energy between them. It can be seen that the surface functionalization has provided additional information during the synthesis of carbon materials for the treatment of organic compounds.