An infrared CO2 gas sensor was developed based on space-double-beam structure. First,the structure and working principle of the optical probe were introduced.A new-type gas cell was presented and the optimal size of the gas cell were calculated in detail. Then, the linear mathematical model of the sensor was established based on Lambert-Beer Law, the principle of space-double-beam optical structure and the working model of an infrared light source in turning on and off periodically. Finally, the unknown coefficients in the mathematical model were calculated with Least Squares Method. The characteristics of the infrared sensor were tested in static state, tested results show that the values obtained from the experiment is close to the true values and the accuracy of the sensor is 0.024 2% when measurement range is 0~3%. The system can work continuously more than 30 days without the disturbance from the temperature signal and background optical signal, it also can work at 5~40 ℃ and on different background conditions. These results conclude that the sensor can satisfy the system requirements of lower-power-consume, higher-precision and stability.