Abstract: The early debonding damage of fiber reinforced polymer (FRP) composite strengthened concrete structure tends to be closed state, which can not be detected and located by traditional linear ultrasonic technology. Based on the continuous laser excited narrow-band ultrasonic technology combined with the nonlinear ultrasonic second harmonic method, a method of detecting the debonding damage of FRP composite strengthened concrete was proposed. This method uses intensity modulated laser technology to excite narrow-band ultrasonic surface wave on the surface of the reinforced structure. Under the ultrasonic disturbance, according to the contact nonlinear theory of the spring model, the debonding damage in the structure is in the boundary. The opening and closing effect will be produced on the surface, and the location of peel damage will be detected by acoustic nonlinear second harmonic response. Based on the results of simulation and experiment, the feasibility of this method for early debonding damage detection of FRP composite strengthened concrete structure was verified, and the non-contact and high sensitivity damage detection of FRP composite strengthened concrete structure was realized.