Abstract: The reaction mechanism of thermal degradation of polystyrene has been studied theoretically using density functional theory B3LYP/6-311G(d) method. The main products of PS thermal degradation are styrene, followed by aromatic compounds such as toluene, α-methylstyrene, ethylbenzene and dimer. It is reported that the thermal degradation of PS mainly includes the homolytic reaction of carbon-carbon bond, β-cleavage reactions, hydrogen transfer reaction and free radical termination reaction. Base on the above reaction, the pathways was designed, and the theoretical calculation and analysis were carried out. Furthermore, the geometrical structure of all the reaction molecules was optimized for the reaction process and the reaction frequency was calculated, which were obtained the standard kinetic parameters and thermodynamic parameters of each thermal degradation path. The results of calculation show that the major formation mechanism of styrene is the chain-end β-cleavage reactions of free radicals, the producing of dimer mainly depends on the intermolecular 1, 3 hydrogen transfer reactions, α-methylstyrene is generated through intermolecular 1, 2 hydrogen transfer and β-cleavage reactions, the radicals of benzyl and phenethyl captured hydrogen atoms to form toluene and ethylbenzene, respectively. Kinetic analysis exhibit that the energy barrier for the formation of styrene was lower than that needed for the generating of other products, so styrene was the main thermal degradation product. This is consistent with the relevant experimental results.