A cycloid pinwheel reducer includes a housing, two cycloid pinwheels, a plurality of rollers, an output flange, and a first transmission assembly. The housing forms an inner teeth ring in an inner wall of the housing. The two cycloid pinwheels are received in the housing. Each cycloid pinwheel forms a meshing portion. The rollers roll in position between the housing and the meshing portions. The output flange is non-rotatably connected with the cycloid pinwheels. The first transmission assembly includes a first transmission gear, a second transmission gear meshed with the first transmission gear. The second transmission gear includes a base body, a second teeth portion positioned on the base body, a first eccentric portion, and a second eccentric portion. The second teeth portion is meshed with the first transmission gear. The two cycloid pinwheels is sleeved on the first and second eccentric portions respectively.