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The development of pore structure in coal reservoirs determines the adsorption performance of coal gas. The pore structure of three coal samples with different degrees of metamorphism, namely long-flame coal, coking coal and anthracite, was tested by low-temperature liquid nitrogen adsorption experiments. The pore structure was quantitatively characterized based on fractal theory, and the effect of the pore structure on the methane adsorption properties of coals with different degrees of metamorphism was analyzed in depth in combination with the methane isothermal adsorption experiments. The results showed that the degree of metamorphism and the pore fractal dimension D1 showed a “bathtub” variation, and a linear negative correlation with D2; while the specific surface area and pore volume of the coal sample were positively correlated with the adsorption constant a. The larger specific surface area and pore volume, the stronger the adsorption capacity of coal. With the increase of pore fractal dimension D1, the adsorption constant a presents an approximate linear growth trend. It can be seen that the less smooth the pore structure of coal, the larger the specific surface area will be, so that the ultimate methane adsorption capacity of coal will also increase.