Objective To explore the influence of nanoparticles on thermal stability of polyimide film and the influence mechanism. Methods In-situ polymerization was adopted to prepare the PI/ TiO2 nanocomposite film, and the thermal stability and micro-structure of composite film were tested through DTA-TGA and SEM. The composite film microstructure was observed after heating to explore the mechanism of heat-resistant composite film by differential thermal analysis. Results The results showed that the nano-particles increased the thermal stability of film. The temperature of 5% composite film increased by 19. 3 ℃ and 20. 7 ℃ under weightlessness of 10% and 50% respectively, compared to the pure film. The peak temperature of film with 5% component was 637. 8 ℃ , increased by 40. 1 ℃ compared to the pure film. Through comparing the pattern of reaction peak, it can be found that the reaction peak of pure film was wide and smooth, while that of three kinds of composite films became narrow with the increase of components with higher and sharp pattern, indicating that the PI composition was fiercer and concentrated in the composite film. Conclusion The study indicated that the nano-particles hindered the thermal motion of PI and slowed down the void spread of film in the process of decomposition. The nano-particles formed the skeleton structure in the matrix, improving the thermal conductivity and rigidity of film. PI molecules at the interface of organic-inorganic phase interface had better thermal stability.