The invention provides a switch power supply suitable for a high-overvoltage-grade high-altitude place. The switch power supply comprises a DC bus voltage input, an open-loop isolation DC/DC circuit and a closed-loop isolation DC/DC circuit; an input end of the DC bus voltage input is connected with a DC bus of electric power equipment, and an output end is connected with the open-loop isolation DC/DC circuit; and an output end of the open-loop isolation DC/DC circuit is connected with the closed-loop isolation DC/DC circuit, and an output end of the closed-loop isolation DC/DC circuit outputs an output voltage Vo. The switch power supply adopts two stages of the isolation DC/DC circuits, the former stage is in an open-loop state, and the latter stage is in a closed-loop state; the switch power supply can achieve a large electric gap and a creepage distance, and then the switch power supply can be suitable for the high-overvoltage-grade high-altitude place; and the switch power supply can compatibly and simultaneously take electricity from the DC bus and an external input power supply, and the switch power supply is ensured to be capable of continuously providing power in certain conditions.