Abstract: In view of the problem that the gas in each coal seam group cannot be accurately measured when the coal seam group was combined drainage, according to the distribution law of CH₄ volume fraction of gas component and carbon isotope, and based on the law of conservation of chemical composition mass, the calculation model of mixed source ratio of coal seam group combined gas extraction was established, the comparative tests were carried out in Xiaotun Coal Mine. The results show that the CH₄ volume fraction of each coal seam gas is sorted from large to small with 6_D, 6_M and 6_U, the CH₄ carbon isotope value of the gas is distributed between -3.6% and -3.2%, and the carbon isotope value of each coal seam gas is sorted from heavy to light with 6_D, 6_M and 6_U. The ratio of coal seam mixture was calculated on the basis of carbon isotope principle when the 6_U, 6_M and 6_D coal seam were combined extraction, the ratio of 6_U coal seam mixing was between 10.82% and 24.54%, the ratio of 6_M coal seam mixing was between 57.81% and 69.58%, and the ratio of the 6_D coal seam mixing was between 5.88% and 31.37%, which was basically consistent with the results of stratified measurement investigation. It shows that the calculation of the proportion of gas combined extraction of coal seam group and gas mixed source based on carbon isotope is scientific and accurate, and the results provide new research ideas for the evaluation standards of gas combined extraction of coal seam group.