Abstract: A pure polyurethane film and a modified polyurethane film containing ultraviolet light absorber UV-531 were prepared by spin-coating. Accelerated aging tests of the films exposed to ozone atmosphere, UV radiation, and UV/ozone atmosphere, were performed by using self-designed aging equipment. The aging behaviors of the films exposed to different atmospheres were evaluated by color difference, yellowness index, UV-Vis spectra, and Fourier transform infrared spectra. There is distinct synergistic aging effect between ozone-aging and UV-aging on both PU and M-PU films. The yellowing resistance properties of the polyurethane film in UV radiation atmosphere, UV/O₃ atmosphere, and short-time ozone atmosphere can be improved by adding 0.2570 UV-531 into the film. The urethane structures in the two films decrease with increasing exposure time during exposure to the studied three atmospheres.