External bonding of fiber reinforced polymer (FRP) plates or sheets, because of their advantages, such as high strength to weight ratio and good resistance to corrosion, has become a popular technique for the strengthening and upgrading of structurally inadequate or damaged reinforced concrete (RC) structures. Interface debonding failure is one of the most common failure modes of the FRP strengthened RC structures. In this paper, the damaged concrete constitutive model is established and the effects of crack on the interfacial stresses of RC beam strengthened with CFRP are investigated. Longitudinal stress in the CFRP, shear stress in the adhesive layer and the first principal stress in the concrete at the crack tips of the retrofitted RC beams with cracks at different locations are analyzed. The results show that when cracks locate at the loading position, the longitudinal stress in the CFRP is the largest and the tensile failure of the CFRP is the most likely occurred.