Based on the adsorption potential theory, this paper processes and analyzes the isothermal adsorption data at different temperatures of four coal samples with different ranks. The research results indicate that the adsorption characteristic curve of methane adsorbed on the coal sample is a single curve, and the mathematical expression of the methane adsorption capacity, temperature and pressure is achieved from the characteristic curve. According to the calculating routine and method presented, the adsorption capacity at other equilibrium conditions can be calculated from the adsorption data of one temperature. The results of this paper are a significant contribution to the research on the storage mechanism of the coalbed methane.