A method is presented for atmospheric CO2 balloonsonde measurement. Based on Beer-Lambert law and non-dispersed infrared(NDIR) technique, an experimental device is designed and developed. An infrared LED is used to produce suitable light source, and infrared LED detectors and a proper designed electronic circuit 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 12 days on ground with EC9820 analyzer. Result shows that the trend of the measurement data of 12 days is consistent, and standard deviation is 9.835 ppm, which basically satisfies the precision demand for atmospheric CO2 measurement. The feasibility of CO2 measurement has been validated.