Abstract: In order to study the difference of gas diffusion characteristics of soft and hard coal, soft and hard coal samples from Huainan Dingji, Anyang Longshan Coal Mine with different degree of metamorphism were selected. And the dynamic diffusion process of gas adsorption desorption was simulated in the laboratory. The dynamic change characteristics of gas diffusion coefficient of soft and hard coal were compared. The pore structure parameters of soft and hard coal were determined by pressure mercury test to analyze the mechanism of difference of the desorption and diffusion parameters of the soft and hard coal. Results show that the diffusion coefficient of soft coal is obviously higher than that of hard coal at the initial stage of desorption, and faster decay over the time. The decreasing function which is monotonically decreasing is appeared for gas diffusion coefficient with the diffusion time throughout the diffusion time, and eventually tends to be stable, and the diffusion coefficient shows the characteristic of time variation. The volume of mesopores and macropors of soft coal is obvious higher than that of hard coal, which makes the gas diffusion coefficient of soft coal obviously higher than that of hard coal. The difference of gas diffusion paths, which are stored in different pores and positions, is the reason for the change of gas diffusion coefficient of soft and hard coal with time.