Context <p>Polyimide (PI) and epoxy resin will age by hot corrosion and long-term high temperature, losing the heat insulting property and forming tremendous potential dangers. In order to evaluate the thermal properties accurately and detect the potential damage of them, the pyrolysis processes of them were studied. The results show that the main products of PI are CO₂ and CN· at high temperature, and their formation are both associated with the break of C-N bond in the imide rings. With the increase of the temperature, the number of CN· increases, but the number of CO₂ decrease. Among several reaction path of the PI productions, the <i>p1</i> is the lowest activation energy and can form CO₂. The bond dissociation energies of C-N bond in <i>p3</i> and <i>p4</i> are higher than <i>p1</i>. We also investigated the pyrolysis process of the epoxy resin. The results show that the main products of epoxy resin are H₂, CH₂O, H₂O, and CH₄ at 1300&#xa0;K, and the H₂ is generated by the collision of the hydrogen atoms (<i>p1</i> path), the CH₂O is generated by the partial decomposition of the C₂ or C₃, which can form the epoxy groups on the ends of the epoxy resin.</p> Methods <p>Reactive force field (ReaxFF) molecular dynamics simulations were used to study the pyrolysis of PI and epoxy resin. The initial structures of PI and epoxy resin were constructed using Material Studio software, followed by geometry optimization to achieve the most stable configuration. Pyrolysis simulations were performed using the large-scale atomic/molecular massively parallel simulator (LAMMPS). The simulation employed NPT ensemble (0.1&#xa0;MPa, 298&#xa0;K) to adjust the system density to 1.0&#xa0;g/cm³, and NVT ensemble for pyrolysis Calculations with a time step of 1&#xa0;fs and total simulation time of 1&#xa0;ns. Temperature was controlled using the Bersenden method, with key simulation temperatures including 1300&#xa0;K (epoxy resin) and 2800–3800&#xa0;K (PI).</p>