Abstract: In order to study the adsorption mechanism of CH₄ and H₂O adsorbed on different ranks of coal, the macroscopic macromolecule stability configuration of different coal rank was calculated on the B3LYP/6-31G basis group by Density Functional Theory (DFT) in quantum chemistry method. Adsorption energy and charge transfer were calculated when CH₄, H₂O and CH₄ and H₂O coexist. The results show that coal adsorption of CH₄ is physical adsorption, and the adsorption capacity of CH₄ is enhanced with the increase of coal rank. The adsorption of H₂O takes the form of hydrogen bond, in which the oxygen-containing functional group is hydrogen bond donor, H₂O occupies adsorption sites of CH₄ when H₂O and CH₄ coexist, leading to the adsorption of H₂O, the adsorption of CH₄ and the increase of free CH₄. In this paper, the adsorption mechanism of methane and H₂O adsorbed by coal was improved from the molecular level, which laid the theoretical foundation for exploiting coalbed methane by heat injection.