A high sensitivity interrogation system of fiber Bragg grating (FBG) sensor with composite cavity fiber laser is proposed and experimentally demonstrated. The FBG is used as a sensing grating and the chirped fiber Bragg grating (CFBG) is used as a reference grating. Both are placed at the end of the fiber Michelson interferometer (MI) acts as the two arms mirrors and form a composite cavity of the fiber laser. The longitudinal modes generated by the composite cavity fiber laser are superimposed on each other to generate envelopemodulation of the beat frequency signal (BFS) which depends on the optical path difference between the two arms. The wavelength change of the sensor FBG can be obtained by measuring the BFS drift causedby the arm length change of the MI. In our scheme, the wavelength change of sensor FBG is converted into arm length change by using CFBG and finally into fiber laser BFS drift. The system has a simple structure, low cost, and high sensitivity.