Abstract: According to the diffraction pattern of coal, the conventional method judging coalification degree is to calculate the ratio of the 002 peak height and FWHM. In this paper, the diffraction patterns of nine kinds of coal were sorted out and calculated. The results show that this method is more reliable for the judgment of medium and low metamorphic grade coal, but it is lacking for high rank coal. In addition, the low-angle region (3-10°) of the diffraction pattern of coal, which is calculated by Bragg equation, reflects the micropore information in the coal with a diameter of (0.884-2.94) nm, and its diffraction intensity corresponds to the porosity in coal. Therefore, the function of the coalification degree is defined according to the regulation of porosity variation with the coalification degree. It can be integrated coal information in the low-angle diffraction patterns and 002 peak to determine the degree of coalification. It is proved that this function is more feasible, reliable and accurate for characterization of coalification degree.