This paper analyzed the debonding process of carbon fiber reinforced plastics(CFRP)-steel composite beams through FBG strain response and studied the realtime response and forewarning which played by FBG in the debonding process.The dobonding moment were determined in the experiment through the strainresponse of FBG which were bonded on the CFRP board. Monitoring the position of the beam which debonding firstly and not affecting the mechanical characteristic too much by FBG can give a strainresponse moment.The experiment results show that the realtime features of FBG can play a very well role in monitoring and forewarning.