The work aims to understand the evolution mechanism for mechanical property of polyimide film by far ultraviolet irradiation. Deuterium lamp was used as simulation source of far ultraviolet irradiation, electronic tension tester was adopted to detect mechanical tensile, thermo gravimetric and XPS were used to analyze and test composition, valence bond structure and thermogravimetry. Under far ultraviolet irradiation, tensile strength and rupture elongation of polyimide film decreased firstly, then exponentially increased and finally tended to be stable as the radiant exposure increased. From thermogravimetric analysis, the temperature of far ultraviolet irradiation causing obvious mass loss in polyimide film dropped to 550 ℃ from 575 ℃. From XPS analysis, rupture and crosslink of chemical bond in polyimide film could be caused by far ultraviolet irradiation. In early stage of far ultraviolet radiation, the rupture of C—N, C=O, C—O, C—C bond was caused to decrease mechanical properties of polyimide film. As far ultraviolet exposure increased, the increase of C—O—N bond, the decrease of C—N bond and C—O—C bond and generation of free N mainly led to the increase of mechanical properties of polyimide film as bond crosslinking. Rupture and crosslinking of molecular bond are primary cause for the change in mechanical properties of polyimide film under far ultraviolet irradiation. As the far ultraviolet exposure increases, the molecular bond firstly ruptures and then crosslinks.