In the gas concentration detection process using the characteristic spectrum absorption method, in order to improve the detection accuracy of the gas concentration, it often has to use the high-quality narrowband modulated laser and modulate wavelength to align with the characteristic absorption peaks of measured gas. But by this way, the cost of the laser and system requirements will be greatly increased. To use the existing portable, low-cost semiconductor laser conditions, at the same time it can obtain higher precision, conversion window differential absorption optical structure and the algorithm of differential characteristic absorption ratio was designed. Selection reason of position of the wavelength characteristic was analyzed, and steps to implement the processing algorithm were given. Systematically utilizing the combination method of conversion window and absorption gas chamber, by the method for calculating the ratio of the light intensity response, the light intensity from non-characteristic absorption peak position was divided out. So it achieved a similar detecting effect was achieved that used a narrow-band laser aligned to the feature absorption peak position. Experiments adopted MW-IR-1650 infrared laser, type SSM17-2 stepper motor control module, C30659 infrared detectors, and other devices. In the experiments, different concentrations of methane gas were tested, and experimental results show that the relative error of measurement was less than 2.0% within the range from 200 to 5 000 ppm. In summary, it’s proved that the system has high accuracy and stability.