The requirements and principles of using photoacoustic gas detection technique in fire detection area are discussed and an unusual photoacoustic gas detection system is presented. Common photoacoustic gas detection systems in laboratories are not suitable for fire warning because of their poor real-time performance and high wavelength qualification for the optical setup. Besides, they are too complex and expensive. These limitations can be solved with a different system design in which an enclosed photoacoustic cell filled with CO is used to achieve CO selection instead of optical filters or laser infrared source. Online CO measurement is realized in an exposed absorption path between the cell and the IR source. At first when there is no combustion gas in the absorption path, the CO cell absorbs the light radiation at 4.6 μm and gives an initial photoacoustic signal. Once combustion gases pass through, the contained CO composition can reduce the radiation that reaches the cell at 4.6 μm and accordingly attenuate the initial photoacoustic signal. Thus CO selection in the absorption path is achieved by enclosed CO itself in the cell, and the photoacoustic signal attenuation further indicates the CO concentration in the absorption path.