To study the adsorption and diffusion behavior of methane on coal macromolecules under different moisture contents, the macromolecular model of coking coal was established through elemental analysis and 13 C NMR and X-ray photoelectron spectrum (XPS) experiments. Using this molecular model, the variation of methane adsorption capacity under five different moisture contents (0, 1.4, 2.3, 3.2, and 4.4%) was investigated. The effect of methane adsorption was explained from energy distribution and the radial distribution function (RDF). Finally, the diffusion behavior of methane on adsorption capacity was studied according to the diffusion coefficient. Simulation results indicate that absolute adsorption capacity and saturated adsorption capacity of methane decrease with the increase of moisture content. Compared with experimental results, the relative adsorption error of the saturated adsorption A is 7.49%; the curve trend of the simulation results is basically consistent with the experimental results, which conforms to the law of the Langmuir adsorption curve (type I). From the energy point of view, the methane molecular priority at the adsorption site is between -22 and -20 kJ/mol and the peak value is about -19.4 kJ/mol, which is not better than the adsorption position. Based on the heteroatoms in the model, the influence of the oxygen atom and the sulfur atom on methane adsorption is stronger than that of the nitrogen atom. Through the calculation of the diffusion coefficient, the diffusion coefficient decreases with the increase of the moisture content, which leads to the decrease of the absolute and saturated adsorption capacities. On the basis of this fact, the diffusion coefficient is found to directly affect the adsorption capacity and adsorption characteristics. This conclusion provides a valuable reference and a new way to study similar coal molecules.