Abstract: An optical fiber gas detection system with two gas cells was developed based on the near infrared absorption mechanism of gases. A Light emitting diode is used as a broadband light source. The wavelength of the LED is modulated by the fiber grating and piezoelectric ceramics to obtain the narrowband reflection output light corresponding to gas absorption peak, and then the high sensitivity detection of gas density can be realized by the detection of the second harmonic. The influence of the beam path interference on the detection accuracy, the variation of absorption coefficient with the environment and the fluctuation of the luminous power were eliminated by the ratio of the two secondharmonic signals from both the measurement and the reference gas cells. The detection experiment for CH4 was implemented by the system. Experiment indicates that the measurement sensitivity of the system is 0.2×10-6.