Abstract: It has important influence of structure size of pressure swing adsorption tower on gas concentration gradient change and mixed gas separation effect. Through the establishment of the channelling effect model and wall effect model, the paper analyzes and calculates the equilibrium of the pressure gradient and masters the gas concentration variation characteristics in the radial and axial of adsorption tower, reveals methane pressure concentration gradient variation under the pressure change condition, clears correspondence between methane concentration gradient in adsorption tower under pressure change condition and adsorption tower size, so as to provide technical support for optimal dimension design of pressure swing adsorption tower in nitrogen and methane separation, and lay the foundation to research low concentration coal-bed gas pressure swing adsorption and concentration apparatus.