The invention belongs to the technical field of particle accelerators, and relates to a passive energy reducer device for regulating the energy of particles extracted from an accelerator, in particular to a wheel axle type particle accelerator energy reducing device and energy reducing method which can be used in vacuum. The energy reducer includes a rotary driving mechanism, a rotary disk on which a plurality of energy reducing stop blocks are mounted and a vacuum system. The vacuum system enables the rotary disk and the energy reducing stop blocks to be in a vacuum environment, and the rotary disk is rotated through the rotary driving mechanism, thereby changing the thickness of the energy reducing stop blocks at a beam position to realize energy adjustment. The energy reducer has a compact structure, which is in favor of miniaturization; a magnetic fluid sealing transmission device is adopted, and thus vacuum reliability is high; a wheel axle type structure is adopted, rapid adjustment of different energy can be realized; the energy reducing stop blocks are convenient to dismount, thereby facilitating replacement; and the energy reducing method is a non-superposition type energyreducing method, and has little influence on beam emittance.