An instrument based on Fourier transform infrared spectrometer is developed, which is capable of simultaneously measuring carbon dioxide (CO2) and 13CO2 in a single air sample with higher precision. The temperature and pressure effect on the values of CO2 and 13CO2 are investigated. Both the standard gas and air sample gas are continuously measured using this method. Precision and accuracy are improved by comparing with the measurements of standard gas after correction using the law of temperature and pressure effect on the values of CO2 and 13CO2.