Industrialization and development of the global economy have boosted the production of plastics, leading to an explosive growth in the generation of plastic wastes. This work aims to establish a molecular-level kinetics model to understand the pyrolysis of polystyrene as a representative case for waste plastic. As a huge number of molecules in waste plastics make models extreme complex with stiff equations, the conception of intermediate monomer is proposed to construct the simplified model. The automatically generated network containing 3,008 substances and 15,025 reactions is partially visualized. The simulation results correspond well to the experimental data at different conditions. Through the evaluation, validation, and application of the model, it is clear that the molecular-level model behaves exquisite and requires low computation power but can give an insight into the pyrolysis of polystyrene.