Provided is a flywheel energy storage device including a radial magnetic gear, the radial magnetic gear includes an inner rotor, an outer rotor and N1 first magnetic pole adjustment sheets embedded in a first magnetic pole adjustment sheet base; a disc-type magnetic gear electric motor includes a first stator disc, a first rotor disc, a second stator disc, a second rotor disc and a third rotor disc, the first stator disc with the first rotor disc form a first disc-type electric motor, the second stator disc with the second rotor disc form a second disc-type electric motor, and a disc-type magnetic gear is formed by the first rotor disc, the second rotor disc and the third rotor disc, and the first disc-type electric motor, the second disc-type electric motor and the disc-type magnetic gear are couple with one another to form the disc-type magnetic gear electric motor.