Major issues of the acceleration grid power supplies (AGPS) for negative ion based neutral beam injector (N-NBI) contain generation of DC high voltage and suppression of surge energy into the beam source at breakdown. An inverter type power supply has been designed as the 200 kV/25 A AGPS for CFETR N-NBI prototype. The circuit parameters and the structure of the diode rectifier with oil-impregnated insulation is designed. The maximum electric field intensity in rectifier tank is 9.8 kV/mm by 3D electrostatic field simulation. To prevent damage of the beam source from fault energy at high voltage breakdown, a core snubber made of Fe-based nanocrystalline soft magnetic materials is designed. The eddy current ratio is determined to be 1 by the simulation and test verification on a snubber prototype. The designed snubber can limit the fault current within 1kA in simulation, and the surge suppression is more efficient through the distributed core snubber.