A permanent magnet speed governor having a fixed magnetic gap. The permanent magnet speed governor has an outer magnetic rotor connected to a drive shaft and an inner magnetic rotor connected to a driven shaft, at least one outer permanent magnet being evenly distributed along the circumferential direction of the inner circumferential surface of the outer magnetic rotor, at least one inner permanent magnet being evenly distributed along the circumferential direction of the outer circumferential surface of the inner magnetic rotor, two magnetic pole sides of the inner permanent magnet being respectively fixed to an iron yoke, another two sides each being provided with a magnetically conductive body, one end of the inner magnetic rotor being provided with a magnetic circuit regulator used for moving each magnetically conductive body along the axial direction. Adoption of the fixed magnetic gap structure reduces the difficulty of assembly.