As the proportion of new energy sources in the grid increases, the insulation performance of reactors in ultra-high voltage direct current (UHVDC) transmission systems becomes a critical challenge. Internal heating and local discharges lead to severe aging and degradation of inter-turn insulation materials, significantly affecting equipment safety. This study employs ReaxFF reactive molecular dynamics simulations to reveal the thermal degradation mechanism of polyimide (PI) insulation materials at 2600 K. Under these conditions, PI main chains experience chemical bond breakage and changes, resulting in the generation of small molecules such as CO, H2O, and H2, ultimately leading to insulation failure. Simulation results validate the effectiveness of ReaxFF in capturing atomic-level reaction details and provide a deeper understanding of the material’s chemical nature. Using molecular dynamics simulations, the PI monomer model was optimized, and a periodic amorphous cell system containing PI chains with a degree of polymerization of 3 was constructed. Energy minimization ensures structural stability of the simulation system at 298 K, consistent with experimental density measurements. The study finds that over time, PI chains progressively degrade, with the main types of decomposition products remaining stable, consistent with experimental observations. These findings deepen the understanding of the micro-mechanisms of aging and degradation in inter-turn insulation materials of dry-type air-core reactors.