Permeability, one of the most significant parameters for CBM, is affected by many influencing factors. In this paper, an improved fully coupled permeability model was proposed to investigate the influence of the effective stress, adsorption, diffusion and variable Klinkenberg's effect on the evolution of permeability. Then, the permeability model is validated by comparing with the experimental data in previous paper. Then, a series of cases were carried out to analyze the influence of various factors on the permeability evolution. Results showed that when the adsorption layer thickness is considered, the effective pore radius exhibited the remarkable difference. With the rising pore pressure, the adsorption layer thickness and sorption-induced swelling deformation increase, leading to the reduction in effective pore radius and thus the depression in gas permeability. Diffusion and Klinkenberg's effect exhibit the remarkable enhancement effect on the permeability, especially at the low pore pressure conditions. It is noticed that compared with the previous constant Klinkenberg's factor, this work constructed a variable Klinkenberg's factor, which is affected by the effective pore radius. Finally, we constructed a governing equation for gas transport in coal seam to reveal the pressure evolution and the contribution of different influencing factors to the total permeability.