A new tunable molecular filter is developed based on gas molecular filtering technology, through deep investigation of the infrared spectrum characteristics of carbon monoxide (CO) gas. The filter combined with differential absorption spectroscopy technology, through two independent spectral channels, the background noise is effectively eliminated, thereby the detection accuracy of CO gas is enhanced. To verify the performances of the system, a series of experiments are designed such as concentration and gas chamber length verification experiments. The precise control for the infrared transmittance of CO gas is realized through changing the gas concentration in gas chamber and the length of the gas chamber. The strong absorption peak of CO at a wavelength of 4.6 μm is particularly focused on. Using this tunable molecular filter, it is possible to achieve a transmittance over 80% for CO at the 4.6 μm peak wavelength, which provides favorable conditions for subsequent imaging application.