Aiming at the problems of high cost and complex structure of optical fiber sensing systems using traditional self organizing fiber lasers, a optical fiber Michelson interferometer(FMI) sensing demodulation system based on the strong feedback of a commercial laser is proposed. The system adopts a commercial Fabry-Perot cavity semiconductor laser diode(FP-LD) as the light source, directly driven by direct-current power supply, and form strong feedback on the FMI after reflection and interference. Multi-longitudinal mode laser beat frequency demodulation technology is used to realize the sensing demodulation of FMI. The change of FMI optical domain interference light intensity is directly converted into the change of electric domain beat frequency envelope. The change process of beat frequency signal with temperature and the sensitivity of sensing system with different FMI arm length difference are studied. The experimental results show that when the arm length difference is 0.45 m and 0.40 m, the sensitivity is 325 kHz/℃ and 358.33 kHz/℃ respectively.