The applications of the carbon fiber reinforced polymer (CFRP) material to the strengthening and retrofitting of reinforced concrete (RC) structures is becoming more and more extensive. The interface debonding at the end of CFRP sheet between CFRP and concrete due to stress concentration usually leads to a brittle failure. It is of great significance to detect the interface debonding effectively. In this study, a novel interface debonding monitoring technology for CFRPstrengthened RC beams under flexure is proposed on the basis of the stress wave measurement using piezoelectric lead zirconate titanate (PZT) patches as actuators and sensors. The CFRPstrengthened RC beam is tested under fourpoint flexure, and the measurement response of the PZT patches from the initiation of cracks to the final failure is analyzed and the damage index is defined. The test results show that the proposed interface debonding monitoring method can effectively monitor the occurrence and development of the interface debonding of CFRPstrengthened RC beam and provide assistance for early warning of the conditions of CFRPstrengthened RC structures.