Abstract: With the rapid development of the nuclear industry, the amount of waste to be treated and monitored increases, and the potential emissions will also increase, and C-14 needs to be monitored. C-14 produced by nuclear facilities is mainly combined with CO₂, so sensitive monitoring of ¹⁴CO₂ gas is of great significance. In this paper, cavity ring-down spectroscopy (CRDS) is used to detect the CO₂ absorption spectrum by selecting the center wave number of ¹⁴CO₂ absorption line around 2209.11 cm⁻¹. A CO₂ detection system based on mid-infrared cavity ring-down spectroscopy is developed by using a quantum cascade laser in mid-infrared band, an optical resonator, and a reasonable optical path design and gas pipeline design. Based on the developed system, different concentrations of CO₂ (0.5%-1.3%) were detected, and the experimental data were fitted and analyzed, and the results were found to be consistent with the theoretical formula, which confirmed the feasibility of the developed mid-infrared cavity ring-down spectrum system. The system has successfully realized the accurate measurement of different concentrations of CO₂ gas in the mid-infrared band, the absorption spectrum of ¹⁴CO₂ was measured by mid-infrared cavity ring-down technique, which provides an important technical support and foundation for the future study of laser spectral detection of ¹⁴CO₂.