The on-line detection is necessary for the characterization of coal, a kind of fossil resource widely applied in industry. Terahertz(THz) spectroscopy is an efficient method for on-line determination and can act as the supplementary tool of the traditional methods in this field. The refractive index and absorption coefficient spectra of nine coal materials are initially calculated from 0.1 THz to 1.5 THz based on the terahertz time-domain spectra. Cluster Analysis(CA) is then employed with the input of refractive and absorption spectra over the whole range from 0.1 THz to 1.5 THz. According to the CA dendrogram and the tested physical properties of coals，a conclusion can be drawn that the carbon content is the main factor of refractive effect, and hydrogen as well as nitrogen play a key role in absorption，respectively. Therefore, the analysis of the similarity and diversity can be realized by THz technique combined with CA. This combination can support the constant and on-line monitoring in coal industry.