The prepulse voltage of the FlashＩ and its reduction ar e analysed. The prepulse voltage between the cathode and the anode has been measured with the capacitive voltage detector, and a new improved equivalent circuit model is proposed. The important effect of direct capacity coupling betw een the intermediate cylinder and the end of diode is pointed out. The prepulse voltage simulated with this model come s closer to the measured result. The scheme of reducing prepulse voltage further is designed．