A braking device includes a flywheel rotatably mounted on a brace, a circular rotor deposited on the flywheel, and a stator fixed to the brace for combining with the rotor. The stator includes first and second magnetic poles. The first magnetic pole is larger than the second magnetic pole. The first and second magnetic poles are arranged into a ring in an interlaced manner. Each of the first and second magnetic poles has a coil. The braking device has a magnetic control unit mounted on a lateral of the brace near a rim of the flywheel and configured to generate magnetism in response to electric currents generated by the coils, so as to change a rotatory load of the flywheel. With the interlaced arrangement of the large and small magnetic poles, the braking device can have the rotor rotating with less cogging torque, and in turn reduced noise.