Carbon dioxide (CO₂) accounts for about 0.04% of the atmospheric composition and is one of the major greenhouse gases. With the development of industrial society, anthropogenic CO₂ emissions are increasing every year, which undoubtedly aggravates global warming. Therefore, it is of great significance to monitor the CO₂ concentration in the atmosphere to manage the CO₂ emission scientifically. In this paper, a simple atmospheric CO₂ gas detection equipment based on Off-axis Integrated Cavity Output Spectroscopy (OA-ICOS) was constructed using a 1.573 μm distributed feedback diode laser. Firstly, the CO₂ detection system was built and optimized. In this paper, the absorption spectrum of CO₂ at 6 358.65 cm^-1 with a line intensity of 1.732×10^-23 cm^-1/(molecule·cm^-2) is selected, and then the CO₂ direct absorption signal of 400×10^-6 in the sealed cavity is measured, and the experimental results show that an effective optical range of about 2.4 km is realized in the lens reflectivity of 99.98% and 60 cm cavity length. There is a large amount of residual cavity mode noise in the transmission signal acquired in a single time, and the system noise can be reduced by averaging calculation. The experimental results show that the optimal value of the averaging number is at 1 000 times, and at this averaging time, the relative error of the absorbing area is averaged at 7.67×10^-3 and the STD is 5.87×10^-3. Second, the performance of the CO₂ detection system was analyzed. By measuring CO₂ gas from 400 to 2 000×10^-6 with an interval of 200×10^-6, the linearity of the OA-ICOS system was obtained to be 0.996, and the maximum value of the STD of the absorbed area at each volume fraction was 3.23×10^-3 (1 200×10^-6) and the minimum was 2.34×10^-3 (1 800×10^-6). By measuring the CO₂ direct absorption signal in the sealed chamber for a long time, the optimal integration time of the system was obtained from the Allan curve analysis to be 98.3 s, and the minimum detectable limit was 0.63×10^-6. The system response time is able to analyze the response speed of the system to the change of the measured value, which is an important parameter to characterize the performance of the system. In order to obtain the system response time, the gas control valve of the mass flow meter was repeatedly switched to alternately pass 2 000×10^-6 of CO₂ and N₂, and the ventilation time was 180 s. The experimental results showed that the average value of the system rise (CO₂ pass) response time was 50 s, and the fall (N₂ pass) response time was >180 s. The fall time was longer than the rise time due to the fact that the CO₂ density was larger than that of N₂, and CO₂ was deposited at the bottom of the cavity, resulting in CO₂ residuals inside the cavity after 180 s of N₂ pass-through (~300×10^-6). Finally, the application of indoor CO₂ measurement was carried out on the OA-ICOS system. The results of 96 h of continuous indoor CO₂ detection experiments show that the system can better reflect the activities of laboratory personnel, and some details of indoor CO₂ changes, such as the intermittent stay of experimental personnel near the experimental platform to carry out other experiments, can also be monitored, which verifies the reliability and stability of the measurement device, and at the same time, provides a practical scientific basis for the management of indoor CO₂ emissions. Conclusion, the CO₂ detection equipment proposed in this paper is characterized by simple structure, high sensitivity and robustness, which is suitable for the detection of CO₂ in the atmospheric background, and lays the foundation for the further development of CO₂ detection instruments for atmospheric CO₂.