Abstract: For a high current diode composed of a small disk type ceramic insulator water/vacuum interface, the design of radial insulation structure is presented. According to the theory of vacuum flashover and the design rules for radial insulation, a “cone-column” anode outline and the cathode shielding rings were adopted. The electrostatic field along the insulator surface was obtained by FEA simulation. By adjusting the outline of the anode and the shielding rings, the electric fields were well distributed and the field around the triple junction was controlled. A high voltage test was carried out on a water isolated spiral pulse forming line(PFL) accelerator with 200 ns pulse width, and the results show the diode can work stably in 400 kV, 200 ns conditions.