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A self-developed gas desorption and diffusion experimental system was used to conduct isothermal methane gas desorption and diffusion experiments, and the pore structure of coal samples was analyzed. A new mathematical model for gas diffusion in coal particles was established, and the diffusion coefficient was calculated using the new model. The influences of particle size and the adsorption equilibrium pressure on the methane gas diffusion rate, gas diffusion quantity, and diffusion coefficient were analyzed and discussed. The results show that under different adsorption equilibrium pressures and different particle sizes, the diffusion rate and quantity vary substantially. The larger the particle size, the slower the initial gas diffusion rate and the longer the time required to reach desorption equilibrium. The larger the adsorption pressure, the greater the initial gas diffusion rate and the larger the quantity of diffusion gas accumulated. However, the coal particle size has little influence on the amount of gas diffusion. It was found that as the particle size decreases, the diffusion coefficient in the first diffusion phase is significantly reduced, which decays with a negative exponential function over time. However, the adsorption equilibrium pressure has no obvious influence on the diffusion coefficient. In addition, the methane gas diffusion process was divided into three phases, namely, phase I, rapid diffusion phase (0–10 min); phase II, slow diffusion phase (10–100 min); and phase III, smooth diffusion phase (100 min); and the diffusion mechanism corresponding to each phase was analyzed. These research results provide a theoretical and practical reference value for gas prevention and control in coal mines.