Shanxi Debao has affluent coalbed methane (CBM) reservoir in China. Understanding its characteristics, especially the diffusion characteristics of natural gas in the coal seam, is the key data of CBM development in this block. In this paper, a method of measuring the gas diffusion coefficient of coal and rock by the adsorption method is proposed by using the experimental method. The calculation model for calculating the gas diffusion coefficient of coal and rock is established according to the adsorption amount, and the diffusion correlation coefficient under different pressures, different water saturation and the different temperatures is established. The results show that it is feasible to calculate the diffusion coefficient of the gas in coal and rock according to the adsorption capacity of coal and rock. With the increase in the pressure, the diffusion coefficient increases, the water saturation increases. While the diffusion coefficient decreases, the temperature increases, and the diffusion coefficient increases. The results calculated by the model could provide guidance for the future development of Debao coalbed methane reservoir in Shanxi Province, China.