The shape of an arc shield in a vacuum interrupter (VI) is varied for the development of insulation. Through the experiment, the flashover voltage is estimated, and the computer-aided finite element method (FEM) is applied to observe the electric field distribution when the impulse voltage is applied. The three parameters upper side length of the floating arc shield, lower side length of the floating arc shield, and slope of the end plate of the arc shield are selected as design variables to find the optimal condition for minimizing the maximum electric field. The response surface methodology (RSM), combined with the full factorial design (FFD) is applied as an optimization method, where the FFD is used to decrease the number of iterations of the experiment.