Abstract: A 4 MV water pulse forming line switch that would be used in a water pulse forming line machine was designed based on the experiments of the 1 MV water self-breaking switch. The designed breakdown voltage is 4 MV with a current about 600 kA. The water self-breaking switch is made up of electrodes, a prepulse suppression plane and joint parts. The water switch is at the joint of pulse forming line and water insulated tri-plate transmission line. The gaps of the switch are arrayed at the input section of the tri-plate transmission line linearly. The discharge current of each gap will be tested by Rogowski coil while the input and output voltages of the switch by D-dot probes. Present 2 D and 3 D models of the electrostatic field analysis are useful tools for the design. There are many differ