Abstract: In the light of the adsorption equilibrium theory, adsorption velocity of adsorbing coalbed methane may be speeded up by injection gas into the coalbed, and the rate of exploitation of methane and the rate of recovery may be lifted. The paper educes calculation formula of the largest theory recovery amount and the rate of recovery when exploitation of methane by the natural decompression and injection gas. By means of these formulas effect can be evaluated, and sorts of injection gas may be selected. It is calculated out on the basis of these formula and statistics datum that the largest theory rate of recovery of methane in our country is 44% when the nature decompression exploitation, but the largest theory rate of recovery is 76% and 54% separately when injection CO₂ and N₂.