This paper aims to describe methane adsorption in coal under the conditions of high temperature and high pressure, as well as quantitatively decipher the change rule of energy in the isothermal adsorption process. The isothermal adsorption test was carried out with four groups of middle-rank coals from the Linxing area with different degrees of metamorphism. The impacts of the degree of deterioration of coal, temperature, and pressure on adsorption were analyzed with regard to the adsorption amount, adsorption potential, and adsorption space. Additionally, the energy change during the adsorption of methane by the coal was considered. The results show that the coal adsorption capacity hinges on the degree of deterioration of the coal, as well as the pressure and temperature. Additionally, the impact of temperature upon coal methane adsorption under depth conditions is highlighted. Like the adsorption space, the adsorption potential is an important parameter used to quantitatively characterize the adsorption ease and adsorption capacity; furthermore, the adsorption potential of millipores exceeds that of mesopores, as they are capable of offering a larger specific surface area for adsorption. The total decrease in the surface free energy during adsorption increases as the pressure increases; simultaneously, the increase rate is fast and then slow. The total decrease in the above-described free energy diminishes as the temperature escalates. Under the same pressure, the total decrease in the aforementioned free energy increases as the reflectance of the specular body of the coal increases. The decrease in the aforementioned free energy at each point of pressure lessens as the pressure grows; notably, when the pressure is comparatively low, the reduction is very fast. As the pressure escalates continuously, the decrease speed is slow. Regarding the effect of pressure and temperature upon adsorption, the adsorption gas volume of coal exists in a conversion depth from 1200 m to 1500 m; at the same time, the impact of pressure upon adsorption is dominant up to this depth. Additionally, beyond this depth, temperature gradually comes to have the greatest impact on adsorption.