A novel set of prestressing system for U-straps of FRP was developed for shear strengthening of RC beam to restrain debonding failure, realize active reinforcement and achieve efficient utilization of FRP.It included self-locking anchorage, angle steel and device for connecting and tensioning. The single point loading tests of 5 RC beams with full-sized rectangular section were carried out, among which one remained unstrengthened, and the other four were strengthened in shear using unbonded prestressed U-straps of CFRP.The feasibility of the technique was verified and the influence of prestress on the behavior of RC beams shear-strengthened with unbounded prestressed CFRP U-straps was discussed.The experiments show that the system can provide reliable prestress for U-straps of FRP and transform the conventional debonding failure of FRP to the tensile failure.Prestressing can obviously inhibit the occurrence and development of main diagonal crack and the stiffness degradation after cracking. The level of prestress has a significant influence on the inclination angle of the main diagonal crack and the number of U-straps of FRP directly involved in shear resistance, but it has a little effect on the effective strain of these fibers.But the shear-strengthening effect cannot keep becoming better with the increase of the prestress.In this experiment, the maximum shear capacity of the strengthened beams is reached with a prestrain of 0.001.Based on the test data, a new expression was proposed for shear capacity of the strengthened beams using unbounded prestressed U-straps of CFRP, in which the influence of the vertical average prestress on the inclination of the main diagonal crack was considered.The predicted capacities are in accordance with the test results.