The global climate change caused by the increase in the concentration of greenhouse gases( GHG) in the atmosphere is the focus of attention in the world. Among all sources of emissions,the emission of carbon dioxide from point sources is a major factor in the greenhouse effect. The Intergovernmental Panel on Climate Change ( IPCC) will directly measure emissions as the highest level of GHG emissions inventory statistics to improve the accuracy of data statistics. In order to achieve accurate measurement of emissions,direct measurement of point source concentration is critical. An off-axis direct absorption spectroscopy technique combined with a long optical path gas absorption cell and a scanning laser frequency method was used to establish a relative method for accurate measurement of carbon dioxide concentration. The absorption spectra of the ( 30012) ( 00001) R20e transition line at 6362.5 cm~(-1) in the range of 293 K and 0 ~ 13 kPa have been measured. The concentration of 15%,35%,50%,and 75% of carbon dioxide mixture was obtained by comparison with the absorption area of pure carbon dioxide,and a detailed uncertainty analysis was performed on the measurement results. The results have a good consist with the results obtained by the balance weighing method,the expanded relative measurement uncertainty is below 0.7%( k = 2).