Abstract: In order to study the difference of gas absorption between soft coal and hard coal with different grain sizes, we use high-pressure volumetric method to measure different particle sizes of soft and hard coal adsorption capacity and adsorption constants and to fit the isothermal adsorption curve. The difference of adsorption performance between soft and hard coal was analyzed through simulation experiments. The study shows that with the decrease of the particle size, the adsorption constant of soft coal and hard coal both increases, the increasing trend decreases with the decrease of the particle size. The soft coal will has the maximum adsorption capacity when the particle size is less than 0.1 mm, but the hard coal produces the maximum adsorption capacity when the particle size is 0.20 mm to 0.25 mm. When the size is 1 mm to 3 mm, the adsorption isotherm curve of soft coal is always above hard coal. But the adsorption isotherms of hard coal and soft coal have a point of intersection, the adsorption amount of hard coal overtakes soft coal when the particle size is 0.20 mm to 0.25 mm and less than 0.1 mm.