Abstract: Provided are a magnetism modulating ring structure, a magnetic gear assembly, and a compound motor. The magnetism modulating ring structure includes a plurality of modulating units and a plurality of connection parts. Each two adjacent modulating units are connected by one of the plurality of connection parts to form the magnetism modulating ring structure, and the magnetism modulating ring structure is arranged within an annular gap enclosed by a first rotor structure and a second rotor structure. A groove structure is formed on one side of the modulating unit facing the first rotor structure, and two sides of the groove structure respectively form a curved boot-like part, and an edge of the curved boot-like part facing the second rotor structure has a curved contour line parallel to flux lines passing through an inside of the curved boot-like part.