Planetary reducer

The invention provides a planetary reducer which is composed of a first-level planetary gear mechanism and a first-level novel small-tooth-difference gear mechanism. The planetary gear mechanism is composed of a sun gear and a planetary gear. The small-tooth-difference gear mechanism is composed of an eccentric shaft, an outer gear and an inner gear casing. The inner gear casing is fixed, the eccentric shaft is fixedly connected with the first-level planetary gear to serve as an input shaft of a second-level small-tooth-difference gear mechanism, the outer gear conducts planar motion under the combined effect of the eccentric shaft and the inner gear casing to enable the outer gear to revolve around the axis of the inner gear casing and rotate reversely around the self axis, the rotating motion of the outer gear is output through a W mechanism, and the planetary reducer can be connected with a joint portion of an industrial robot.