The utility model discloses a planetary gear speed reducer. The planetary gear speed reducer is composed of a high-speed level and a NGW type low-speed planetary gear transmission mechanism. A fixed axis gear is adopted in the high-speed level for carrying out transmission, the high-speed level comprises a plurality of spline shaft gears ZA and a big gear with internal teeth and external teeth, the spline shaft gears ZA are evenly arranged around the big gear, and the spline shaft gears ZA are meshed with the external teeth of the big gear respectively. The internal teeth of the big gear ZC are meshed with drum-shaped external teeth of a sun gear ZZ of a low-speed planetary gear through a floating duplicate gear coupler ZD. The input end of each spline shaft gear is provided with a high-speed hydraulic motor, and the high-speed level of the planetary gear speed reducer can achieve multi-point input. The NGW type low-speed planetary gear transmission mechanism is high in efficiency, small in size, simple in structure, convenient to manufacture, large in transmission power range, and small in axial size.