Abstract: Methane is an inflammable and explosive gas and a major constituent of mine atmospheres, natural gas and gas fuels. Becuse gas explosion is a big problem in mining industry,real-time and on-line detection of methane has a critical function both for safety assessment and for control of combustion processes. Based on the mechanism near infrared spectral function both for safety assessment and for control of combustion processes. Based on the mechanism of near infrared spectral absorption of methane, a system using absorbent type optic-fiber for high sensitivity methane detection with DFB LD is demonstrated. Light source modulation harmonic measurement is presented in this paper. The ratio of the fundamental and second-harmonic signal presented can be used for eliminating optical interference. The mathematical model of gas concentration harmonic measurement is built up. The result of methane concentration is also shown. Optical fiber is used as transmission medium,thus the sensor part and measurement circuit can be isolated completely.The safety of the systems is inhanced.