The invention discloses a rotary speed reducer output shaft fixing system. An output shaft is sequentially provided with a baffle ring, a bearing I and a bearing II from inside to outside, wherein the inner collars of the baffle ring, the bearing I and the bearing II are installed on the output shaft; the output shaft is installed in a machine base; the outer collars of the bearing I and the bearing II are assembled on the machine base; the outer end of the output shaft is sequentially provided with an adjusting pad, a planet carrier, a baffle plate II and a baffle plate I; the adjusting pad, the planet carrier, the baffle plate II and the baffle plate I are connected sequentially through screws; the inner collars of the baffle ring, the adjusting pad, the bearing I and the bearing II are in close fit with the output shaft and are relatively fixed; the baffle plate II and the planet carrier are in close fit and are relatively fixed; and the bending moment of the rotary speed reducer output shaft is born by the bearing I and bearing II, and the torque moment is transmitted by an involute spline between the output shaft and the planet carrier. The rotary speed reducer output shaft fixing system has the advantages of simple structure and manufacturing convenience, the bearing clearance can be adjusted by controlling the tightness of the screws, the situation of too large or too small bearing clearance is avoided, and the service life of the bearings is prolonged.