Abstract: In order to study the adsorption equilibrium time of gas adsorption constant (adsorption equilibrium 7 h in first point, and then 4 h when the pressure is higher than 0.5 MPa) in the criterion of "Determine method of methane adsorption capacity in coal (high-pressure volumetric method)( MT/T752-1997)", the method is used to make the anthracite achieve real adsorption equilibrium. The COMSOL software is adopted to simulate the gas adsorption diffusion process to investigate the adsorption equilibrium time of anthracite. The results show that the adsorption equilibrium 4 h is not enough to make gas adsorbed in the pores of anthracite completely, the adsorption equilibrium time should be extended when testing the gas adsorption constant of anthracite. Meanwhile, for the same coal (anthracite) including soft coal and hard coal, the soft coal porosity is more developed that hard coal. If the gas enters the smaller micro-pores, the molecular diffusion resistance will be bigger and bigger, and the equilibrium time required will be longer and longer. Therefore, the adsorption equilibrium time of soft coal is longer than the time of hard coal under the same adsorption pressure.