Abstract: The degradation of polystyrene in supercritical toluene was studied at 310—370℃, 3.6—5.2 MPa by using a novel fast reactor. Supercritical solvents have unique properties such as excellent heat and mass transfer. Compared with thermal degradation without solvents, supercritical degradation obtains high conversion in a very short reaction time. The supercritical degradation of polystyrene is a typical free radical reaction mechanism, including chain fission, disproportionation, radical recombination, β-scission, intermolecular abstraction and intramolecular abstraction. Based on the reaction mechanism, the process of polystyrene degradation in supercritical toluene was modeled at the mechanistic level by using population balance equations formulated via the method of moments. The predictions of the degradation conversion of polystyrene were compared with experimental results, and the agreement was good.