The invention discloses a novel joint speed reducer. The novel joint speed reducer comprises a left side fixing shell, a hollow lead screw, a connecting shaft, a left end movable supporting base, a first angular contact ball bearing, a first deep groove ball bearing, a cylindrical shell, a second angular contact ball bearing, a right side movable supporting base, a third angular contact ball bearing, a bearing gland, ball screws, a feeding nut, a movable nut, a first gear, a second gear, a right end fixing shell, an adjusting inlaid strip and a ball frame. The device is simple in structure, a ball screw pair can be pre-pressed and pre-tightened, the transmission gap is eliminated, the theoretical backlash difference can be close to zero, and the joggling problem caused by the fact that when a robot works, rigidity is not enough is avoided from the source; and meanwhile, machining of a ball screw pair spiral race can economically achieve the micron order machining precision, the transmission precision is high, the transmission efficiency is high, and the manufacturing cost is low.