Abstract: In linear induction accelerator, Z-pinch and other large pulsed power devices, the gas-filled spark gap switches as key components are widely used. Unreasonable insulation system may bring excessively high partial electric field and accumulated charges to the gas-filled spark gap switch. The surface flashover would occur while the gas-filled spark gap switches are working under long time or high frequency high voltage pulse. The surface flashover will directly affect the pulsed power devices. Hence, we discussed the mechanism of accumulated charge influence on surface flashover by finite-element electric-field analysis of gas-filled spark gap switch insulation system. The surface electric field strength of insulator and electrodes were reduced by optimizing the structure of insulator and shapes of electrode surface. The results show that: the electric field strength in anode triple junction was decreased from 9.4 kV/mm to 1.5 kV/mm, the electric field strength in cathode triple junction was decreased from 2.95 kV/mm to 0.98 kV/mm, the maximum electric field strength in insulator surface was decreased from 10.8 kV/mm to 4.95 kV/mm. The electric field distribution of optimized insulation system was more reasonable than that of original system, and the probability of surface flashover resulted by accumulated charges would be reduced.