Carbon Dioxide (CO₂), as the most crucial greenhouse gas, significantly influences the regulation of Earth's climate and ecosystems. Accurately detecting trace CO₂ concentrations is vital for assessing the greenhouse effect. Gas molecules exhibit strong absorption in the mid-infrared range, which is advantageous for highly sensitive trace gas detection. This paper describes the design of a compact CO₂ monitoring instrument using an interband cascade laser (ICL) at 4.219 μm as the light source, paired with Tunable Diode Laser Absorption Spectroscopy (TDLAS). The instrument was calibrated with various standard gas concentrations and, using Allan variance analysis, achieved a detection limit of 16.6×10^-9 with an integration time of 8.7 s, meeting the requirements for sensitive CO₂ measurements in atmospheric monitoring. Additionally, the instrument was employed to monitor atmospheric CO₂ over eight consecutive days and was benchmarked against the commercial LGR instrument, confirming the reliability of our miniaturized CO₂ monitor and the effectiveness of combining mid-infrared light source with TDLAS technology for precise atmospheric CO₂ measurement. The findings offer a technical reference for high-precision CO₂ concentration detection, while the system's simple structure, compact size, low power consumption, and lightweight design are ideal for developing CO₂ monitoring capabilities on airborne platforms.