A set of planetary gears (C), consisting of three equally divided gears combined on a tray, is supported by an output shaft, an inner ring gear, and a sun gear (B) driven by external power at the center of the ring gear, which is tightly connected to the gearbox housing; When the planetary reducer is driven by the input side power to drive the sun gear, it can drive the planetary gear to rotate and follow the trajectory of the inner gear ring to revolve around the center. The rotation of the planetary gear drives the output shaft connected to the tray to output power.