Abstract: In order to solve the problem of gas measurement in combined drainage of close outburst coal seams, taking Nenghua Xiaotun Coal Mine in Guizhou Province as the engineering background, the method of combining carbon isotope method and stratified measurement is proposed to determine the proportion of gas drainage in any coal seam group, so as to provide the basis for calculating residual gas content in coal seam gas extraction standard evaluation. Based on the law of carbon isotope mass conservation, the calculation model of mixed source gas in different coal seams is established. The single coal seam and mixed source gas in 6 upper, 6 middle, 6 lower, 6 upper + 6 middle, 6 upper + 6 middle + 6 lower were collected from representative sites. The carbon isotope was measured by Isoprime 100 isotope ratio mass spectrometer in the laboratory, and the model was solved by MATLAB. At the same time, the layered measurement method is used for field test. In the mixed source gas, the 6 upper coal seam accounts for 17%, the 6 middle coal seam accounts for 61%, and the 6 lower coal seam accounts for 22%. According to the field practice, the deviation between the calculated residual gas content and the measured gas content is less than 5% in the evaluation of the standard of 16 middle coal seam in 14 working face, which indicates that the comprehensive determined gas source ratio is true and reliable.