Based on the massive applications of FRP-reinforced structure technology, the time-varying patterns of interfacial stresses of FRP-strengthened RC beams under sustained load are investigated in this study. A finite element (FE) model was developed to analyze the external FRP strains, and the correctness and usability of the FE method were verified by comparing the predicted results with the test results. For the FRP end area, both the FE method and the analytical method were applied to conduct investigations considering the spew fillet of adhesive and the secondary loading of the reinforced structures. For the area near the intermediate concrete crack, the analytical method was used to conduct calculations. The results show that the concrete creep leads to the increase of interfacial stresses with time, and the adhesive creep leads to the relief of stresses. This conclusion is applicable to both areas mentioned above and is not affected by the spew fillet of the adhesive and the secondary loading of the reinforced structures. In the FRP end region, the FE results and analytical results are in good agreement.