Based on the mode-hopping of single mode diode laser, two different currents drive the laser alternately, the laser emits alternately two beam lights whose wavelength is differently, but closely. The first beam light＇ s wavelength is at the absorption peak of methane. The second beam light＇ s wavelength is a little shorter than the first one, which is not absorbed by methane, and it will serve as a reference light beam. The double beam will be converted into an electronic square wave after passing through the gas cell. The intensity of the double beam will correspond to the top and the bottom of the square wave respectively. The top of the square wave will vary if the methane concentration varies, but the bottom of the square wave will not vary because the intensity of the second light does not vary while methane concentration varying. Thus the amplitude of the square wave is proportion to the methane concentration. Because the two wavelengths are very close, the scatters or the losses caused by dust are almost the same after they passing through the methane, and they can be eliminated by mathematics. The result of experiment shows that the method can reach the sensitivity of 0. 05% even without using a phase-locked amplifier and the absorption light path is only 10 centimeters.