A multistage pericyclic gear reducer includes an input shaft, an input external-teeth gear coaxially coupled to and rotatable with the input shaft, a first middle planet ring gear coupled between a first driver ring gear and a first driven wheel. The first driver ring gear meshed with the input external-teeth gear. The multistage pericyclic gear reducer further includes a second middle planet ring gear coupled between the first driven wheel and a second driven wheel, an output external-teeth gear extended through and meshed with both the first middle planet ring gear and the second middle planet ring gear, and a central output shaft coupled to the output external-teeth gear.