In order to achieve high accuracy and high stability of methane gas concentration in complex environment, this paper proposed the half-wavelength scanning detection method which using sawtooth modulation based on spectral absorption principle, and this method greatly improved the performance index of the system. In the process of detection, the method of sawtooth modulation realized wavelength modulation and intensity modulation of Distributed Feedback(DFB) laser, scanned Lorentizen absorption linear of methane gas. And it improved the accuracy of detection through eliminating the measurement error and the influence of laser wavelength drifting. Due to the symmetry of methane absorption spectra, relative to full-wave scanning would appears sine curve after differential circuit and could not establish the relation between absorption intensity and digital value, the half-wave scanning would appear concave curve after differential circuit, and could establish the relation between absorption intensity and digital value. The experimental results show that this method could accurately measure methane gas concentration, and the measurement range of methane gas concentration is 0%-10%, the accuracy increased from 100 ppm to 10 ppm(1 ppm=10-6), stability is increased from 0.3% to 0.01%. The system was not affected by temperature in complex environment and has high accuracy, good stability, and strong anti-interference capacity after improvement.