For reinforced concrete beams strengthened with prestressed carbon fiber reinforced polymer (CFRP) plates, design methods are derived for determining the allowable plate area or initial tension control stress of CFRP plate to achieve a targeted value of ductility. The derivation of equilibrium and compatibility equations for a rectangular cross section is presented. Base on the test of 2 reinforced concrete beams strengthened with prestressed CFRP plates, analytical results are compared with experimental results. Comparisons show that the design methods have good applicability. It can provide a reference for engineering application of strengthening design.