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The surface charge distribution under impulse voltage is measured using a static capacitance probe. A probe with very small charge leakage is designed. The condition of surface charge accumulation under impulse voltage is analyzed, and it is concluded that micro discharges in the gas near the insulator surface such as the corona caused by free and fixed metal particles is usually a prerequisite condition. The dynamic equation describing the relationship between surface charge density and the applied voltage is established, and the process of surface charge accumulation under impulse voltage is analyzed. Theoretical analysis and experimental results show that the decrease of wave front time of the impulse voltage can result in an increase of surface charge accumulation. A GIS spacer is used to investigate the influence of charge accumulation on the flashover characteristics. It is shown that the 50% impulse flashover voltage can be reduced by 23.4%, and the lower limit of the V-t characteristics can be lowered drastically if the polarity of the surface charge is opposite to that of the applied voltage.