The steel beam is strengthened by prestressed CFRP sheets, which can increase capacity, reduce distortion and improve the utilization of CFRP cloth. Through the adhesive layer, the CFRP sheets and steel beam work together. Therefore, the interface is the weak link components, the adhesive layer and the physical and mechanical properties of CFRP fabric is an important factor for interfacial shear stress. Based on the elastic theory, a prestressed CFRP cloth and steel interface shear stress differential equation is established, and a calculation formula for interface shear stress is given. The calculation results show that, along the steel beam, interfacial shear stress distribution of CFRP is found with non-uniform, which increases the fastest growing at the end, and grows with increase of initial prestress.