A novel high voltage DC power supply for neutral beam injector (NBI) is proposed. An acceleration power supply of NBI will experience frequent load short-circuits owing to the spark downs in the load. The output DC voltage of the proposed scheme can be quickly disconnected from the short-circuit load. In addition, the DC voltage is applied to the load immediately after the fault has been cleared without causing any voltage overshoot. The proposed scheme has the characteristics of a simplified structure, reduced cost and volume. Experimental results are presented to verify the usefulness of the proposed scheme.