The instrument adopts gas filter correlation infrared absorption method. The basic working principle is: CO can absorb the infrared radiation of a specific wavelength (4.67 μ m), and the absorption intensity has a certain relationship with the concentration of Co. The detection method is as follows: the infrared from the infrared light source passes through the CO and N2 filters in the rotating filter wheel, and then enters the light chamber through the narrow-band interference filter. When passing through the co filter in the filter wheel, the infrared ray is absorbed, so the CO in the sample gas can not be reabsorbed in the light chamber, thus forming a reference beam with fixed light intensity; when passing through the N2 filter in the filter wheel, the infrared ray is not absorbed, so it shines on the sample gas in the light chamber, and the CO in the sample gas absorbs the infrared radiation, and the weakened infrared radiation forms the measuring beam. The reference beam and measurement beam are detected by infrared detector. According to the light intensity ratio of the two, the concentration of CO in the sample gas is calculated by Lambert Beer law.