Based on the characteristic absorption spectrum of CO2 at 4.26 μm, a micro gas detection system of CO2 concentration is designed with the hermetic extractive White cell and thermoelectric sensor. Practical measurement was carried out and the concentration of CO2 were retrieved. Correlation coefficient and detection limit can reach 0.99973 and 1 ppm, respectively. The system has simple structure and runs stably. It can avoid the complex structure of the optical disk modulation method, and reduce the instability impacts on detector due to its open structure.