The invention provides a planetary reducer, and belongs to the technical field of machinery. The problem that the accumulative installation errors of an existing planetary reducer are large is solved.The planetary reducer comprises an input end shell and an output end shell; the input end shell is internally provided with an input shaft coaxial with the input end shell, and one end of the input shaft is provided with a sun gear; the output end shell is internally provided with a planet carrier, a planet gear is fixed to the planet carrier and engaged with the sun gear and an inner gear ring arranged on the output end shell; the outer wall of the input shaft is sleeved with a first bearing matched with the input shaft, the planet carrier is sleeved with a second bearing and a third bearing, and the second bearing is located between the first bearing and the third bearing; the outer end of the planet carrier is provided with an output shaft which is integrally connected with the planetcarrier and stretches out of the output end shell, and the output shaft and the input shaft are coaxially arranged; and the inner end of the planet carrier is provided with a supporting part which extends in the axial direction and is in a barrel shape, and the inner wall of the supporting part is used for supporting and being matched with the outer circle of the first bearing. Through the planetary reducer, accumulative installation errors can be decreased so that the operation precision can be improved.