Abstract: A lower prepulse high power density water dielectric self-breaking switch was investigated on FlashⅡgenerator. The switch contained two or four gaps, a prepulse shield and supporting structure. The discharge current and the input or output voltages of each gap were measured by a Rogowski coil and CuSO4 resistance dividers respectively. The test and analysis methods of the experimental waveform were presented. The amplitudes of the prepulse of the water switch were significant. It was considered that the prepulse effect time of the water switch was equal to the breaking delay time. The prepulse current or prepulse voltage could be used to provide timing data to measure the delay time, the jitter of the same gap and the first-to-last switching time difference. The dI/dt signal of a gap coul