Abstract: Insulator vacuum surface flashover is one of the main factors restricting the development of high voltage, high current and the miniaturized pulse power technology applications. In the vacuum environment of PTS, the insulation performance is greatly affected when soft X-rays radiate the insulating material. Therefore, in order to improve the insulation properties of cross-linked polystyrene material in PTS and obtain a deeper understanding of surface flashover mechanism, the radiation effects of soft X-rays on cross-linked polystyrene were studied, influence of different irradiation times on the surface flashover performance was observed. The experimental results show that the surface flashover performance of samples without irradiation is better than the soft X-ray irradiated samples in vacuum, and the surface flashover performance of 14 times irradiated samples is a little better than 2 times irradiated samples in vacuum. At the same time, the mechanism of the soft X-ray radiating insulator surface is used to explain the experimental results very well.