Abstract: In order to investigate the influence of granularity on gas diffusion law, the gas adsorption and desorption experiment system was built which could accommodate massive coal samples, to simulate the gas diffusion process. Taking granularity change as the main factor, the gas diffusion law of long flame coal was studied on dry coal samples with different damage types at constant temperature (30 ± 1) ℃ and constant pressure 0.74 MPa. Combined with the pore structure of soft coal and hard coal, the differences were analyzed about the diffusion amount and the diffusion initial speed of different coal samples.