As a key component, the Blumlein switch is widely used in large pulsed power devices such as high-current electron linear induction accelerators. The insulator in the main switch plays the role of isolating water or oil and gas. When the equipment is subjected to high voltage pulse for a long time or high frequency, the gas side of the insulator will flashover along the surface. The occurrence of surface flashover seriously affects the reliable operation of linear induction accelerator. In view of this, electric field simulation calculation is carried out on the insulation structure of the Blumlein main switch, and the electric field distribution on the surface of the insulator and the electrode is effectively regulated by optimizing the geometric structure of the insulator and the shape of the electrode. At the same time, different insulators with different configurations were trial-produced, and flashover along surface was studied under standard lightning wave pulse conditions. The results show that, the minimum and maximum surface flashover voltages of the optimized insulator are about 35.9% and 37.2% higher than that of the original insulator respectively.