The invention discloses a magnetic coupler which comprises a power input shaft, connecting flanges, permanent magnet outer rotor assemblies, cage type inner rotors, a power output shaft and a mechanical speed adjustment device, wherein the permanent magnet outer rotor assemblies have hollow cup-shaped structures, are sleeved outside the cage-type inner rotors and realize transmission of torque under the coupling action of an air gap magnetic field between the permanent magnet outer rotor assemblies and the cage-type inner rotors; the mechanical speed adjustment device can change axial air gap areas between the permanent magnet outer rotor assemblies and the cage-type inner rotors according to a lever theory, thus realizing stepless speed variation for load rotating speed. According to the magnetic coupler, a method for designing the number of pole pairs of the permanent magnet outer rotor assemblies is provided on the basis of a minimum torque ripple rule, so that the thickness of yokes and the rotating inertia of the coupler are reduced. The cage-type inner rotors can adopt structures which are the same as or similar to that of the conventional motor cage rotor iron core. The magnetic coupler disclosed by the invention can be coaxially connected with a load and can be used as a belt wheel adapter matched with a belt wheel.