Abstract: The experimental data of low temperature liquid nitrogen adsorption for coal sample with different metamorphic degrees was researched with multi-fractal theory. Then the relationship between pore multi-fractal characteristics of coal samples, fractal parameters, pore parameters and metamorphic grade was discussed. The results show that, the pore size distribution of different rank coals has obvious multi-fractal characteristic. With the increase of metamorphic degree, the specific surface area ratio of micro-pores in coal samples increases gradually. The pore section that has the highest level of specific surface area will decrease with the increase of metamorphic grade. The smaller the pore size in coal sample, the more obvious the heterogeneity and the larger pores distributed evenly. There was no significant correlation between the connectivity and metamorphic grade of coal pores. Both the singular index a₀ and spectrum width △a had a positive correlation with metamorphic grade, but the parameter R_d had negative correlation with metamorphic grade. In other words, the higher the metamorphic degree, the more uneven the pore size distribution in the coal sample, and the lower the share of the number of small probability subset in the coal sample.