Carbon dioxide （CO₂） is the most important greenhouse gas in the atmosphere， characterized by high concentrations with minimal annual fluctuations. Therefore， high-precision monitoring of its concentration is an important link to realize the objective of the "double carbon." In this study， a CO₂ gas sensing device， with detection sensitivity in the ppb level， was constructed based on continuous-wave cavity ring-down spectroscopy. The CO₂ absorption line with a central wavelength of 6 251.760 cm^-1 was selected. Moreover， a quartz glass Fabry-Perot resonant cavity with ultra-high fineness （>300 000） and temperature and pressure control modules with good performance were designed in the system. The changes in gas temperature and pressure in the cavity during 24 h are less than 0.07 ℃ and 15 Pa， respectively. The Allan variance result shows that the system can obtain a detection limit of 0.7×10^-12 cm^-1 at an optimal integration time of 303 s. For carbon dioxide， the detection limit corresponds to a minimum detectable concentration of 1.6×10^-9. The linear correlation coefficient of the system's response is 0.999 94 over a wide range of CO₂ concentrations. Finally， an observation of the atmospheric CO₂ was conducted for 2 days with a system response time of 10 s. The results are in good agreement with the monitoring data from the commercial instrument （Picarro， G2401）， and the deviation between the two devices is less than 6‰ after excluding the interference from human exhalation. With its simple structure， low cost， and extremely high sensitivity， the system exhibits a broad application in the field of trace gas monitoring.