Abstract: To research the gas migration laws in coal, the non-constant gas diffusion coefficient model of coal was established firstly and then adsorption and desorption and unsteady-state diffusion properties of the coal mass with different particle sizes were analyzed by the improved desorption experiment device, which was used to verify the above model. It was found that the diffusion coefficient at different time during the coal gas diffusing could be obtained and the unsteady-state gas diffusion properties would be acquired after the establishment of non-constant gas diffusion coefficient model. The initial desorption rate of coal samples is very fast and then gradually decreased; the smaller the coal sample particle size is, the greater the total desorption and desorption rate of gas is, and the shorter the needed time of coal sample closing to desorption equilibrium is. Under the condition of same gas pressure, the effective gas diffusion coefficient decreases with the increasing of particle size and the gas desorption and diffusion experiment results verify the high reliability of the above model.