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In this paper, a new organic insulation system is proposed to improve the surface-flashover performance in vacuum by inserting permittivity-gradient multilayer organic composite material between the electrode and the dielectric. Simulation technique was used to calculate and analyze the electric-field distribution of this new insulation system. The result shows that the electric-field strength at the vacuum-insulator-electrode triple junction decreases significantly by optimizing the permittivity and conductivity of the inserted layer. On the basis of this analysis, a new improved system of polyethylene composite material-polyethylene-polyethylene composite material was prepared and studied. The permittivity and conductivity of the composite material are controlled by the additive content in the carbon black semiconductive master batch. Moreover, it is indicated in our result that by properly adjusting the permittivity and conductivity through additive content, the surface-flashover performance in vacuum can be greatly improved. The highest first flashover voltage reaches 91 kV and increases by 75% compared with that of the polyethylene standard samples. It is also confirmed that by decreasing the electric-field strength at the vacuum-insulator-cathode triple junction, the vacuum surface-flashover performance can be improved significantly.