The instrument adopts gas filter correlation infrared absorption method. The basic working principle is: CO has the characteristic of absorbing 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. Through the co filter in the filter wheel, the infrared ray is absorbed, and the CO in the sample gas cannot be reabsorbed in the light chamber to form the reference light intensity. Through the N 2 filter in the filter wheel, the infrared ray is not absorbed and irradiates on the sample gas of the light chamber. The CO in the sample gas absorbs the infrared radiation and forms the measured light intensity irradiates on the infrared detector through the light chamber. According to the light intensity ratio of the two, the concentration of CO in the sample gas is calculated by Lambert Beer law.