The concentration of exhaled CO as a biomarker for some diseases has attracted much attention, but CO concentration detectors had issues of low sensitivity and slow response time. Therefore, this paper used a quantum cascade laser with a central wavelength of 4.59 μm and a multi-pass cell of 3.8 m to build a high-sensitivity, fast-response exhaled CO measurement system based on absorption spectroscopy. The CO concentration was detected and analyzed using direct absorption (DAS) and wavelength modulation (WMS) techniques. The linearity of DAS is 0.998, and its detection limit reaches 3.68 × 10^-8. The linearity of WMS detection is 0.998 at CO concentrations below 6×10^-6, with the detection limit reaching 3.00×10^-9. The optimal integration times of 170 s and 250 s for DAS and WMS were obtained by Allan variance analysis, corresponding to detection limits of 2.00 × 10^-9 and 3.00 × 10^-10, respectively. Finally, 14 volunteers were tested for exhaled CO concentrations, and the results show that the system could distinguish between smokers and non-smokers, providing a scientific and effective tool for judging the smoking status of patients in smoking cessation clinics.