Pyrolysis is a promising technology used to recycle both the energy and chemicals in plastics. Three types of plastics, polyethylene plastic (PE), polypropylene plastic (PP) and polystyrene plastic (PS) were investigated using thermogravimetry−mass spectrometry (TG−MS) and reactive force field molecular dynamics (ReaxFF-MD) simulation. The thermogravimetric analysis showed that all three plastics lost weight during the pyrolysis in one step. The thermal decomposition stability is PS < PP < PE. The activation energies and reaction mechanism function of the three plastics were determined by the Kissinger and CR methods. Meanwhile, the ReaxFF-MD combined with density functional theory (DFT) was used to calculate the kinetics, as well as explore the pyrolysis mechanism. The calculated kinetic results agree well with the experimental methods. The common pyrolysis reaction process follows the dissociation sequence of the polymer to polymeric monomer and, then, to the gas molecules. Based on the bond length between the monomers and the bond dissociation energy for different plastics, the required energy for polymer dissociation is PS < PP < PE, which microscopically explains the macro-activation energy sequence and thermal stability. Moreover, due to the retention of aromatic rings in its monomers, PS almost completely converts into oil.