Based on the off-axis integrated cavity output spectroscopy technology， an infrared carbon dioxide （CO₂） sensing system was developed. A distributed feedback laser centered at 1 572 nm was used， and the absorption line of CO₂ at 6 359.96 cm^-1 was selected as the target line. The length of the resonant cavity developed is 60 cm， and the measured effective optical path is 1 200 m. Using the wavelength modulation spectroscopy technique and the direct absorption spectroscopy technique to measure the absorption spectrum of CO₂， the signal-to-noise ratio of the spectral signal of the former was 130， which was better than the latter's signal-to-noise ratio of 80. The wavelength modulation technology was combined with the off-axis integrating cavity technology， and LabVIEW software was used to extract the second harmonic amplitude. Using the equipped gas samples， performance tests such as sensor calibration， stability and dynamic response were carried out. Using pure nitrogen （N₂） for stability test， Allan variance showes that the detection limit of the system is 5.1×10^-6 when the average time is 96 s. When the gas flow rate is 500 sccm， the experimental response time is less than 20 s. The system was used to carry out continuous online real-time monitoring of atmospheric CO₂ concentration for 16.5 hours and detection of CO₂ content in human breathing gas， both showing good performance. The system has the advantages of easy operation， fast response and high sensitivity， and can be widely used in atmospheric environment detection and medical diagnosis in the future.