A new method is presented for atmospheric CO2 balloonsonde measurement. Based on Beer-Bouguer-Lambert law and non-dispersed infrared (NDIR) technique, an experimental device is designed and developed. A infrared LED is used to produce suitable light source, and infrared LED detectors and a proper electronic circuit designed are used to transform light intensity into electrical signals in the experimental device. Calibration method for CO2 measurement is also presented. A comparison experiment is carried out for continuous 24 h on ground with EC9820 analyzer. Result shows that measurement error ranges from -10×10-6 to 10×10-6, and standard deviation is 3.76×10-6, which basically satisfies the precision demand for atmospheric CO2 measurement. The feasibility of CO2 measurement has been validated.