Abstract: Modification of ion beam pulsing circuits of PKU 4.5 MV Van de Graff accelerator is described. In the present design a bunching frequency of 10 MHz with sweeping frequency of 2.5 MHz sine waves are used. For changing repetition rate of beam pulses an adjustable DC voltage of cross-field mass analyzer deflects the beam away from the aperture and a regulated frequency pulse voltage added on the analyzer returns the beam to go through the aperture again. Beam pulses repetition rates are 2.5 MHz,1.25 MHz,625 KHz…..39KHz.