Abstract: The outgassing rate of materials is a critical factor influencing the vacuum level. Polyimide (PI) is widely used in the vacuum field because of its excellent performance, but its surface and interior will absorb and dissolve a large amount of gas, resulting in the outgassing rate of untreated PI up to the order of 10⁻⁶ Pa·m³/(s·cm²). This study focuses on polyimide films and systematically investigates the regulatory mechanisms of ultraviolet (UV) irradiation on their outgassing behavior. By designing multiple control experiments with varying UV wavelengths (185 nm, 254 nm, 365 nm), powers (40 W, 60 W, 100 W), and irradiation times (0−30 min), combined with an outgassing rate measurement system, the effects of UV photodesorption on the outgassing rate of PI were analyzed. The results demonstrate that UV irradiation, through synergistic mechanisms including direct photodesorption, electronic transition-induced desorption, and photochemical desorption, significantly reduces the outgassing rate of PI. Specifically, using a 254 nm, 60 W UV lamp for 30 min of irradiation decreased the PI outgassing rate by approximately 76%.