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Quantitative characterization of multiphase methane and investigation of the methane dynamic adsorption process of coals were performed by a low-field nuclear magnetic resonance (NMR) method. Meanwhile, methane diffusion behaviors during the step-by-step pressurization adsorption process were evaluated by three diffusion models. The results indicate that the transverse relaxation time (T 2 ) spectra of methane demonstrate two distinct peaks of adsorbed methane (P1, T 2 < 2.5 ms) and porous mediumconfined methane (P2) at a low-pressure step (∼1.0 MPa), and the third peak of bulk methane (P3) obviously appears when the pressure >2.0 MPa. The integrated T 2 amplitude of adsorbed methane increases quickly during the first 2 h (>75% of total) and then gradually reaches a maximum value in the last 4 h during the initial pressure step of ∼1.0 MPa, whereas it reaches >90% of total amplitude in 1 h as the pressure is increased step-by-step. According to the strong linear relationship between the adsorbed methane volume and the integrated T 2 amplitude, the real-time methane adsorption volume can be determined, and adsorption isotherms from the NMR method are found to be mostly overlapped with those of the volumetric method. Moreover, the effective diffusion coefficient of the unipore model (10 −6 to 10 −5 s −1 ) coincides with the micropore diffusion coefficient of the bidisperse model and the slow diffusion coefficient of the multiporous model, which is generally 1−3 orders of magnitude less than the macropore diffusion coefficient (10 −3 to 10 −2 s −1 ) and the fast diffusion coefficient (10 −2 s −1 ) or transitional diffusion coefficient (10 −4 to 10 −3 s −1 ). The dynamic changes in diffusion parameters with pressure may be related to the comprehensive effects of methane diffusion mechanisms and coal matrix swelling under different adsorption pressures.