Abstract: Through the test of specific surface area,total pore volume and adsorption characteristic of soft and hard coal,it is found that for the same metamorphic degree,the specific surface area and total pore volume of soft coal is several times to that of hard coal,but the adsorption capacity of soft and hard coal is equivalent. In order to solve this problem,according to the thermodynamic principle and methane adsorption mechanism in coal,adsorption layer thick- ness theory in coal body pore was established to numerically calculate and analyzed adsorption layer thickness ( ad- sorption layer number),which is effected by pore and adsorption equilibrium pressure. At the same time,pore size and pore volume distribution were measured by mercury injection test,and adsorption isotherms of methane in coal was cal- culated. The results show that based on the adsorption layer thickness theory,the methane adsorption layer thickness decreases with negative index relation to pore diameter at the same adsorption equilibrium pressure,which indicates that methane adsorption in coal has different molecular layer. The relation of pore size and its pore volume can be fitted by piecewise functions. Gas adsorption isotherms in different coal samples are calculated based on the layer thickness,and both the change trend and quantify are consistent with the measured results,the error is less than 6. 5% . There- fore,the adsorption layer thickness theory reveals the methane adsorption characteristics of soft and hard coal. Thus,as long as the pore size distribution characteristics of coal is measured,adsorption quantify can be calculated by adsorp- tion layer thickness theory,which provides a new method for the gas content prediction in coal seam.