CO molecules have the strongest absorption peak at 4.6 μm, which was selected as the central wavelength of the gas absorption. Combined with the luminescence characteristics of light source EMS200, spherical mirror chamber open was designed. A mid-infrared differential CO detection system was developed using the double pass structure of single detector. The performance of the instrument was studied by using the standard gas concentration of carbon monoxide in the mixed gas station. The research results reveal that, the resolution of the instrument is 20 ppm, and the limit of detection (LOD) is 18 ppm. The relative error was not more than 8.5% within the low concentration range of 30-1 500 ppm. Compared with the CO detection systems utilizing laser spectroscopy technology, pulsed infrared thermal source used in this system, its performance-cost ratio was high; with open spherical mirror chamber, the light path was simple and easy to implement. So the proposed detector shows potential applications in CO detection under the circumstances of coal-mine, environmental protection and petrochemical industry.