Abstract: The degradation of phosphorus-containing polyimide (PI) films in the simulated atom oxygen environment is investigated using the samples developed in our laboratory. The test results from scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) indicate that the phosphorous and oxygen element contents in the PI film surfaces are increased significantly after the atomic oxygen irradiation; meanwhile, the binding energies are increased at the same time, which suggests that a surface layer of phosphorus oxide is formed to inhibit the further erosion of the underlined PI film surface. Thus, the present PI films exhibit a good atomic oxygen resistance, whose mass loss rates are much lower than that of the Kapton film.