When a laser is disturbed by low-frequency factors (e.g., thermal fluctuations and mechanical vibrations), laser wavelength drift and light intensity fluctuations occur, which affect the measurement accuracy of the system. In this study, a 2.73-μm distributed feedback (DFB) laser is used as the detection light source and a set of CO₂ gas-sensing system based on hollow waveguide fiber is built. The proposed system uses the third harmonic absorption signal (3f) of CO₂ to achieve the frequency stabilization of the laser and then uses the harmonic signal to reverse the concentration of CO₂. In the experiment, the standard CO₂ gas is measured for a long time and the system is analyzed according to four measurement methods: second harmonic (2f), stabilized second harmonic (2f-lock), second harmonic ratio first harmonic (2f/1f), and stabilized second harmonic ratio first harmonic (2f/1f-lock). The system measurement accuracy obtained under the 2f-lock condition is 0.001255, which is 2.4 times higher than that under the 2f condition. The system measurement accuracy obtained under 2f/1f-lock condition is 0.00138, which is 2.34 times higher than that under 2f/1f condition. It can be seen that the 2f/1f-lock method has the longest stable time of 210 s and the lowest detection limit of 4.24×10 ^-5. Comprehensive analyses reveal that 2f/1f-lock is the most optimal among the four methods. Kalman filtering is performed on the measured data under the 2f/1f-lock condition, and the measurement accuracy of the filtered system is 0.0002786, which is 4.95 times higher than that before the filtering.