Abstract: The propagation behavior of interface fatigue crack in reinforced concrete (RC) beam strengthened with prestressed fiber reinforced polymer (FRP) was studied. The mechanical analysis model was established for the interface crack tip in the strengthened beam under three-point bending. The theoretical derivation of stress intensity factor (SIF) for the interface crack was presented base on the mechanical model, and the effect of the prestress level on SIF was discussed. Combined with the experiments on interface fatigue crack propagation rate, a semi-empirical formula was developed for the propagation rate of interface crack in such strengthened beams. Results showed that the propagation of the interfacial fatigue crack has three stages: rapid propagation, steady propagation and instability propagation. The interface crack propagation rate was described in the form of Paris formula before the instability propagation. SIF of the initial crack at mid-span interface reached the maximum and then decreased with the interface crack length. In elastic range of the reinforcing steel bar, the strengthened beam with prestress had large SIF compared with the strengthened beam without prestress.