Abstract: The surface materials, especially, the polymer materials, used in spacecraft operating in low Earth orbit, may be greatly damaged when eroded by the atomic oxygen (AO). A series of the polyimide (PI) composite films are prepared via the physical blending of the PMDA-ODA matrix and the trisilanolphenyl polyhedral oligomeric silsesquioxane (TSP-POSS) additives. The effects of the incorporation of the TSP-POSS additives on the properties of the composite films, including the thermal stability, the optical properties, and the atomic oxygen resistance are systemically studied. The experimental results indicate that the TSP-POSS additives exhibit a good miscibility with the PMDA-ODA poly (amic acid) (PAA), and the homogeneous PAA/TSP-POSS composite solutions with a good storage stability are obtained. The PI/TSP-POSS composite films with loading amounts varied from 0 to 25% are successfully prepared by the high temperature imidization of the PAA/TSP-POSS solution. The composite films exhibit a good uniformity with the loading amount of the TSP-POSS up to 25%. The incorporation of the TSP-POSS additive improves the optical transparency of the derived composite films, with no impairment of the thermal properties of the pristine PI-0 film. Last but not the least, the incorporation of the TSP-POSS can significantly enhance the atomic oxygen resistance of the PI-0 films. During the AO exposure in the ground-based simulation test with the fluence of 4.02×10²⁰ atoms/cm², the PI-25 exhibits an erosion yield of 2.2×10^-25 cm³/atom, which is only 7.33% of that of the pristine PI-0 film.