During the processing of PS by single screw extrusion, the material is exposed by thermo-mechanical and oxidative loads. Between the temperature of 200 and 300 °C the material degradation takes place, and the degradation process is dominated mainly by chain break. In this temperature range PS is processed by single screw extrusion, that’s why a prediction of the molecular weight loss along the screw by a mathematical model is so important. This model must consider the geometry of the screw and the process parameter to describe universally the material degradation. In the past a degradation model was derived successfully for polypropylene homopolymer, was implemented in the single screw design software REX and was verified by experimental investigation. Furthermore, this degradation model is suitable to calculate the molecular weight loss for PS, because there are the same degradation processes as for PP. The transferability of the model with PS was checked by using a test-setup to stress material samples under variation of temperature, shear stress and residence time. The rate of the molecular weight loss of the samples was characterized by the measurement of the solution viscosity. This method of analysis is acceptable because there was no change in the width of the molecular weight distribution and the curves were only shifted to lower molecular weight. This was confirmed by different research institutes and also in this work. The regression equation and the modeling of PS was implemented in the software REX, in which the material degradation could be simulated depending on the process parameters and the choice of different single screw designs. Experimental investigation with single screw extruders with different screw diameter and various screw designs were performed and the molecular weight loss was determined by the solution viscosity to verifiy the simulation model of PS.