The invention discloses a closed cycloid precision gear reducer. The closed cycloid precision gear reducer comprises an input part, a pin wheel housing, an output part and a small-tooth-difference inside-gear reducing mechanism. Wheel pins used for being meshed with a cycloidal gear of the small-tooth-difference inside-gear reducing mechanism are arranged on the inner ring of the pin wheel housing in the circumferential direction. The inner ring of the pin wheel housing is provided with containing grooves which correspond to the wheel pins in a one-to-one mode and are used for containing the wheel pins. The wheel pins make double-line contact with the inner walls of the corresponding containing grooves. According to the closed cycloid precision gear reducer, the double-line contact characteristic exists between the wheel pins and the containing grooves in the pin wheel housing; as the wheel pins make double-line contact with the corresponding containing grooves in the tooth width direction, the bending strength is enhanced, and carrying capacity is improved; meanwhile, due to the increase of the number of contact lines, the achievement of the error leveling effect is facilitated, transmission accuracy can be improved, and then the requirements for machining errors and assembly errors can be lowered. Compared with an existing RV gear reducer, the return difference of the closed cycloid precision gear reducer is smaller; besides, the closed cycloid precision gear reducer is compact in structure, long in service life, simple in machining process and low in manufacturing cost.