In this paper, a development of a non-dispersive infrared (NDIR) CO2 gas measurement system with an optical path length of 31 mm is described. The system improves its accuracy by optimizing the optical path and the light efficiency. Firstly, the ray tracing approach is used to simulate the light path of the chamber, so the light efficiency can be calculated. Secondly, based on the simulation result of the sensor that every different optical path length has its own contribution to the measurement, the optimized optical path length for CO2 measurement in the wavelength of 4.26 μm is 31 mm. Then the CO2 volume fraction measurement system is designed and tested by measuring the standard gas whose concentration is in the range of 20×10-6~1 500×10-6, and the experimental results show that the measurement accuracy of the system can reach 50×10-6.