In the shear strengthened reinforced concrete (RC) beams by externally bonded fiber reinforced polymer (FRP) composite, there are some main shortcomings including low fiber utilization, premature debonding, and stress hysteresis of the FRP, etc. In order to address these problems, this paper independently developed a novel prestressing system for U-shaped FRP sheet in shear strengthening, which consists of three parts: anchor device, tension device, and steel angle. The design method of component size and the prestress construction technology were presented. Furthermore, a prestressing system for 2-ply and 50-mm-wide FRP sheet was designed and manufactured. One bearing capacity test of linear FRP sheet and eight prestressing loss tests of shear-strengthened FRP strips were conducted in this system. The test results show that: the prestressing system can effectively anchor FRP and result in rupture failure; fiber strength increases up to 96.1%; design method of component size is safe and reliable; prestressing construction technology can exert different prestress values to FRP strip based on the uniformity and symmetry of the strip stress; the system is applicable to exert the prestress more than 244.84 MPa to the FRP; and the long-term prestress loss ratio is about 15%.