Abstract: Polyimide films used in multilayered insulation and deployable structures in spacecraft are susceptible to space environmental effects, which would degrade their mechanical properties. The polypyromellitimide films were exposed to the space proton environment in the geo-synchronous orbit under the condition of different exposure durations, flux and temperatures. After the space exposure, the tensile tests were carried out for the samples. There is no marked degradation when the flux is smaller than 3.17×109 p·cm-2·s-1. As the flux becomes much bigger, the tensile strength and the elongation at break decrease approximately linearly. With the increase of the exposure time, the tensile strength and the elongation at break decrease at a lower speed. Higher temperature causes a smaller change in the mechanical properties.