Abstract: In order to study the evolution characteristics of the pore structure of medium and high rank coals, the low-temperature nitrogen adsorption method and the FHH fractal theory model were used to explore the regularity of pore structure characteristics and fractal characteristics of different rank coals. The results show that as the coal rank increases, the BET specific surface area of the coal increases first and then decreases, the BJH pore volume gradually decreases. As for the coal pore size distribution, the number of micro-pores decreases first and then increases, and the number of small and mesopores increases first and then decreases; the value of the FHH fractal dimension D1 can quantify the roughness of the coal surface, which increases with the increase of coal rank. The change of D1 value of the gas-fat coal and lean coal is small, and anthracite stage increases more, the range is 2.256 2 to 2.461 3; the D2 value can quantify the complexity of the pore structure of coal. The D2 value decreases first and then increases with the increase of coal rank, ranging from 2.459 1 to 2.806 5.