In this study, we propose an exhaled CO₂ gas measurement system based on a mid-infrared hollow waveguide fiber. A distributed-feedback (DFB) laser with a central wavelength of 2.73 μm and a 1-m hollow waveguide fiber were used in the proposed system. The exhaled CO₂ gas was measured in real time via calibration-free wavelength modulation spectroscopy (CF-WMS). The linearity with respect to the CO₂ concentration obtained via CF-WMS and the prepared standard CO₂ gas concentration is 0.9999. Further, when the volume fraction is 0%--6%, the maximum absolute error between the measurement results and the volume fractions of the standard gas is 0.01%. The precision of the CO₂ volume fraction retrieved via CF-WMS is 1.01×10 ^-5, and the CO₂ gas detection limit is 1.3×10 ^-6 at an optimal integration time of 26.00 s. Subsequently, the measurement accuracy and measurement sensitivity of CF-WMS and the traditional calibrated wavelength modulation spectroscopy (WMS) were compared. The obtained results demonstrate that the measurement accuracy and measurement sensitivity of CF-WMS are twice and 1.4 times those of WMS, respectively. The background CO₂ volume fraction obtained via CF-WMS is observed to remain stable at approximately 3.8×10 ^-4 during real-time measurement of the exhaled CO₂ gas; further, the CO₂ volume fraction observed immediately after exhalation remains stable at approximately 5.7%.