Abstract: For the further study the influence of igneous rock invasion on coal gas diffusion characteristics, the initial speed of coal gas diffusion was researched under the same metamorphic grade and different particle sizes, scanning electron microscopy and microscopic spectrophotometer is used to make the microscopic observation of the coal sample invaded and not invaded by the igneous rock with the same metamorphic grade. And the carbon dioxide microporous method is adopted to determine the pore characteristics of the two coal samples. The experimental results show that the coal sample invaded by igneous rock has a bigger initial speed of gas diffusion, and the smaller the particle size with the same metamorphic grade, the bigger the influence. The initial speed of gas diffusion has a index relationship with the particle size.