Abstract: Polyimide film was widely used in multilayered insulation and deployable structures of spacecraft，while their mechanical properties can be damaged by space ultraviolet irradiation.The mechanical property and mechanism of polyimide film in near ultraviolet and far ultraviolet radiation was studied and following results can be obtained：with the increasing of near ultraviolet or far ultraviolet irradiation，the rupture elongation and tensile strength of polyimide decrease firstly and then exponentially increase then tend to stable.In the early stage，the decrease of mechanical properties of polyimide films in near ultraviolet is faster than in far ultraviolet irradiation.And the mechanical properties of polyimide films irradiated by long duration near ultraviolet are better than in far ultraviolet environment.The breakage and cross linkage of molecular bond is the major cause of change of polyimide's mechanical property in ultraviolet environments，and breakage is the major origin in early stage of irradiation and cross linkage is for the increase of mechanical property in subsequent ultraviolet irradiation.