The invention relates to a planetary-harmonic composite gear reducer, which relates to a reducer, and aims at the problems that the prior cup-shaped harmonic reducer has small reduction ratio, large axial size, large volume, large mass and the like. A sun gear is engaged with at least two planetary gear shafts and is arranged in an elliptical internal gear ring of an elliptical cam, the at least two planetary gear shafts are engaged with the elliptical internal gear ring of the elliptical cam to form a primary planetary reduction gear train, the elliptical cam is arranged in a flexible ball bearing, the primary planetary reduction gear train and the flexible ball bearing form a wave generator, the flexible ball bearing is arranged in a flexible gear, the flexible gear is arranged in a rigid internal gear ring and is engaged with the rigid internal gear ring under the action of the wave generator, and a planetary gear shaft arranged in a shaft hole of a tie bar is provided with a rolling bearing. The reducer has the advantages of large reduction ratio, small axial size, small volume, light mass and the like, and is particularly suitable to be applied in the fields such as aviation and space detectors, various instruments and meters, and driving units of precision robots and the like.