Coal samples from typical mining areas in China were collected to investigate the impact of gas pressure on the gas diffusion in coals with different metamorphic degrees. The pore characteristics of coal samples were analyzed using the low-temperature nitrogen sorption and mercury pressure methods, the adsorption-desorption diffusion kinetics experiments of coal particles were made under different pressure. The diffusion experiments results were fitted by applying the unipore diffusion model and the bidisperse diffusion model. The results show that the bidisperse diffusion model more accurately describes the whole process of gas adsorption-desorption of coal particles relatively to the unipore diffusion model. The macropore effective diffusion coefficients Dae and micropore effective diffusion coefficients Die were calculated using the bidisperse diffusion model. The analysis shows that the relationship between Dae and pressure is better described by the quadratic polynomial model. In the adsorption process, Dae decreases with the increase of pressure when the gas pressure is smaller than a threshold value, and Dae increases with the increase of pressure when the gas pressure is greater than another threshold point. In the desorption process, Dae reduces with the decrease of the pressure when the gas pressure is larger than a threshold value, and Dae increases with the decrease of the pressure when the gas pressure is smaller than another threshold point. The average macropore diffusion effective coefficient Dae is larger if the critical value of gas pressure is higher. The micropore effective diffusion coefficient Die and pressure are well consistent with the linear relationship. Die increases with the increase of pressure during the adsorption process, and Die decreases with the decrease of pressure during the desorption process. The diffusion characteristic parameter �� remains the same in the adsorption-desorption process of coal samples.