Steel yielding is a major factor driving the mid-span debonding failure in FRP flexural strengthened RC beams. On the other hand, it is generally required to have steel yielding at the ultimate limit state for an appropriate flexural strengthening design. This paper first presents a critical steel yielding zone model for predicting the mid-span debonding in FRP flexural strengthened RC beams. Based on this model, this paper then performs parametric analysis to study the influences of various design parameters, such as a/d ratio, concrete cover depth, internal steel reinforcement ratio, stiffness of FRP, interfacial fracture energy, and interfacial bond stiffness, on the mid-span debonding strains of FRP. The critical steel yielding zone model proves its convenience in incorporating the bond properties of FRP/concrete interfaces, beam geometries, and reinforcing information of steel and FRP into the mid-span debonding strength prediction while avoiding complex bond-slip analysis. In addition, the parametric analysis provides useful information for optimizing the flexural design of FRP strengthened RC beams.