A new generalized method for evaluating the bond slip relationship of the interface between concrete and the externally bonded reinforcement is presented. The method is based on the relationship between the applied force and the displacement of the reinforcement at the loaded end. As no prior assumption on the detailed configuration of the bond slip relationship needs to be made using the generalized method, the obtained bond slip relationship can be regarded as a generalized relationship. The method was then used to evaluate the bond slip relationship of an investigated FRP-concrete interface reported in the existing literature, and the obtained relationship compared with several available bond slip models. It can be found that various bond slip relationships differ vastly even for the same FRP-interface, especially for the maximum bond stress and the corresponding slip as well as the the slip corresponding to the insignificant bond stress. In addition, the bond slip relationship obtained using the generalized method leads to the good prediction of the experimental results in terms of the global force versus displacement curve and strain distribution along the interface.