The structural compositions and group changes before and after the thermal polymerization modification and the thermal polymerization modification with anthracite were investigated with FT-IR spectroscopy. The pyrolysis and polycondensation characteristics of the coal-tar pitch before and after the modification were studied with TG-DTG. And then, the modified mechanism was discussed by using Gas Chromatography-Mass Spectrometer method (GC-MS). The results showed that the aromaticity of the modified coal-tar pitch with anthracite modifier increased significantly. Anthracite promoted the polycondensation reaction in the process of heat treatment of the medium temperature coal tar pitch.