Pyrolysis is recognized as a promising technology for waste plastics management. Although there have been many studies on pyrolysis of waste plastics, there is still a lack of in-depth research on the mechanism of synergistic effect between mixed plastics and the mechanism of product formation. In this paper, based on the pyrolysis characteristics of Polystyrene, Polyethylene, and mixed plastics (Polystyrene/Polyethylene), it is demonstrated that a synergistic effect exists in the co-pyrolysis of Polystyrene/Polyethylene and affects the pyrolysis behavior and pyrolysis products. It was found that polystyrene chain segments containing C=C double bonds, generated from the pyrolysis of polystyrene, initiated the pyrolysis of polyethylene during the polystyrene/polyethylene co-pyrolysis, resulting in the termination pyrolysis temperature of the co-pyrolysis being advanced by 19.8 K. Due to the reduction in the termination pyrolysis temperature by 19.8 K, the average activation energy of the co-pyrolysis was reduced by about 14%. Compared with the weighted values of single-component plastics (Polystyrene and Polyethylene), the actual oil production of co-pyrolysis increased by 9.7% to 89.80%. At the same time, the content of low molecular weight Styrene and Toluene in pyrolysis oil increased by 12.3% and 1.65%, respectively. This study provides a useful and comprehensive reference for realizing the closed cycle of “from plastics to plastics”.