Abstract: A 40kV/20kW switchingmode pulse forming network (PFN) charging power supply was developed by constantcurrent, seriesresonant and softswitching techniques. High pulse to pulse repeatability was achieved even under the condition of high repetition rate and small PFN capacitance. The power supply also featured in high efficiency, small size and the ability of being used in parallel mode. The techniques can be applied to injection/extraction system of circular accelerators, klystron modulators in Linasc and radar transmission. Design of the power supply and PFN charging and discharging experiments are presented in the paper.