Diffusion is the main migration form of coal seam gas. This paper establishes a conceptual diffusion model to describe the microdiffusion process accurately based on the diffusion form and pore scale. First, low-pressure carbon dioxide and nitrogen adsorption experiments were used to obtain the pore structure characteristics of ten coal samples. Combining the experimental results with the literature, the research determines the main diffusion space, i.e., a pore network with a size range of 1.5~100 nm. Second, based on the principles of different diffusion forms, the main diffusion space is further divided into three parts, including a pore network with size ranges of 1.5~50 nm, 50~70 nm and 70~100 nm, where Knudsen diffusion, transition diffusion and Fick diffusion control gas transport. The three parts are connected in series with parallel pores inside each part. Using partition calculation, the average pore diameters of the parallel pores in the three parts are 26 nm, 60 nm and 85 nm, and the equivalent total pore volumes are 6.52×10 -4 ~1.71×10 -2 cm 3 /g, 1.72×10 -4 ~1.77 ×10 -3 cm 3 /g and 1.49×10 -4 ~1.69×10 -3 cm 3 /g, respectively. This shows that the small-scale pores for Knudsen diffusion are more developed than the pores for the other two diffusion forms. Finally, the linear regression method is used to obtain the correlation between the cumulative gas diffusion volume within 30 seconds and the equivalent total pore volume in the three parts of the ten coal samples, which is 0.914~0.925, 0.023~0.261 and 0.032~0.033, respectively, showing that Knudsen diffusion is the primary factor controlling the diffusion process. This research provides a theoretical basis for gas migration under in situ conditions and is of great significance to the extraction of coal mine gas
