Abstract: The prepulse phenomena in the discharge process of accelerator are analyzed in this paper. The prepulse will result in forming plasma, and further affect the diode performance. To remove or reduce this effect, a crowbar tube with cold cathode was adopted in the accelerator. The results show that prepulse voltage decreased from about 150kV to 50kV after adopting crowbar tube. The output power of relativistic backwardwave oscillator increased from 50MW to 250MW under the same condition. The frequency of relativistic magnetron tended to be reasonable obviously too.