The invention discloses a nested planetary reducer converting reciprocating motion into unidirectional motion. The nested planetary reducer comprises a front overrun clutch, a rear overrun clutch, a composite gear, an output end cover, a planet carrier, a deep groove ball bearing, a front outer gear ring, a rear outer gear ring, rear outer-side planet wheels, rear inner-side planet wheels, front planet wheels and an input shaft. When motion is input clockwise, the front overrun clutch is subjected to engagement and drives the output end cover to output motion counterclockwise after primary direction changing. When motion is input counterclockwise, the rear overrun clutch is subjected to engagement, after the direction of a rear nested planetary transmission mechanism is changed twice, motion is output counterclockwise, and finally, the purpose of converting reciprocating motion into unidirectional motion is achieved. The nested planetary reducer integrates direction changing with speed changing, the complexity, caused by a reversing device, of a transmission system is reduced, and in addition, the nested planetary reducer is large in transmission, flexible in structure, high in smoothness and good in precision retaining ability.