Abstract: The methane physisorption is one of the key issues discussed for the accumulation and exploration of unconventional reservoirs. Due to its multidisciplinary nature, a deeper understanding of the detailed adsorption mechanism is crucial for geological applications. By constructing a representative methane − graphite adsorption system, the detailed variation of key parameters during adsorption process with the increase of pressure is further discussed, including excess and absolute adsorption amount, adsorption phase volume, and density. Adsorption phase is confined between lower and upper phase boundaries, within which is the absolute amount absorbed. Surface excess represents the amount of adsorption phase exceeding the bulk phase at the same density, rather than a phase that exists, and the surface excess do not disturb the distribution of bulk phase. In reservoirs, methane is supercritical, the adjacent layer of the surface mainly contributes the excess adsorption. The amount of absolute adsorption, the volume of adsorption phase, as well as the bulk phase rise with the increase of pressure linearly for low pressures, but the increase rate of adsorption phase density begins to decelerate at 4−5 MPa while the bulk density keeps building up. When the increasing rate of bulk density catches up with that of adsorption phase, the surface excess and the volume of adsorption phase reaches their maximum and subsequently decline, the maximum thickness of adsorption phase is slightly bigger than the diameter of methane molecule. However, the absolute amount absorbed keeps rising until it plateaus between 30−100 MPa, during which pressure range the surface excess, upper absorption phase boundary, along with the absorption phase volume decrease with the rising pressure. For pressure higher than 100 MPa, the surface excess would drop to minimum and then increase again because of the continuous densification of the adsorption and bulk phases. For engineering application, a deeper understanding of the intricate characteristics of the adsorption phase would have a significant impact on the calculation of absolute adsorption amount and corresponding phase density, thus enabling a more accurate assessments of the gas in place and productivity.