Needle coke is a highly structured anisotropic carbon material with graphitizing ability, which is the only existing material for the production of graphitized electrodes of arc furnaces. Previous scientific studies have established the improvement of the anisotropic structure of needle coke by polymer modification of hydrocarbon raw materials of different origin with polystyrene. The aim of this work is to establish the mechanism of polystyrene action in obtaining premium grades of needle coke from highly aromatized petroleum feedstock. To prove the mechanism, 1H NMR, 13C NMR, TGA, Raman spectroscopy, and elemental analysis were used. The results of the studies conducted showed that the effect of polystyrene on decantoil consists in modifying the primary structure of the mesophase pitch formed at the stage of liquid phase thermolysis. Polystyrene at the stage of intensive development of the bulk mesophase ensures the introduction into the system of additional active aromatized radicals formed at polymer decomposition. The bridging structures and alkyl radicals formed in the mesophase pitch increase the time interval for plastic mass formation and provide the mesophase with the necessary molecular mobility. Finally, an increase in the degree of structuring of the needle coke during its subsequent high-temperature calcination is ensured.