The invention discloses a prestressing system for a final joint. The final joint comprises a first joint body and a second joint body. A circle of closed annular water-stop belt is arranged on the bonding surface of the first joint body and the second joint body. A plurality of connecting parts are evenly arranged along the upper inner wall and the lower inner wall of the first joint body. Connecting parts of the same number are arranged at the corresponding positions of the upper inner wall and the lower inner wall of the second joint body. Each connecting part on the first joint body and the corresponding connecting part on the second joint body form a connecting part group. At least one prestressed steel cable is connected between the connecting part groups. By adoption of the prestressing system, the water-stop belt can be continuously compressed through the prestressed steel cables, the effect that the water-stop belt can be in a compressed state all the time in the installation process of the final joint and after installation of the final joint is guaranteed, and water can be prevented from entering the final joint; and meanwhile the water-stop belt is evenly stressed, and the phenomenon that the structure of the water-stop belt is destroyed because of impact caused when external seawater pressure is continuously increased during sinking of the final joint is prevented. The prestressing system is simple in structure, convenient to set up and good in effect.