The adsorption/desorption of coalbed gas will result in the fractionation of methane carbon isotope and multicomponent gases. Fractionation can make the possibility of predication of coalbed gas accumulation and reveal the mechanism of injection carbon dioxide to enhance methane recovery. Based on the Polanyi's adsorption potential theory and the adsorption/desorption isotherm data, the fractionations of methane carbon isotope and multicomponent gases are discussed in this paper. Two conclusions can be summarized: (1) the adsorption potential of 13CH4 on coal is higher than that of 12CH4, especially at high pressure. It suggests that the coal is preference for 13CH4 adsorption and postponement for 13CH4 desorption; the fractionation of methane carbon isotope is intense at high pressure. (2) The fractionation of CH4 and CO2 during the process of adsorption/desorption can be generalized into three situations: a. the adsorption/desorption isotherms of CH4 and CO2 on/from coal is nearly parallel, and the adsorption potential of CO2 is equal to or larger than that of CH4. It implies that the coal is always preferable to carbon dioxide adsorption, but the fractionation is slight at medium pressure (1~2.5 MPa); b. the adsorption characteristic curves of CH4 and CO2 are intersectant because of intersection of CH4 adsorption/desorption isotherms. It implies that the coal is preferable to carbon dioxide adsorption at high pressure (>2.5 MPa); c. the adsorption curves of CH4 and CO2 are intersectant because of intersection of CO2 adsorption/desorption isotherms. It shows that the coal is preference for carbon dioxide adsorption at high pressure (>2.5 MPa). The mechanism of injection carbon dioxide to enhance methane recovery is perfectly characterized by adsorption potential theory. It indicates that it is always favorable for injection carbon dioxide to enhance methane recovery at high pressure (>2.5 MPa).