Abstract: In order to accurately determine the gas influx ratio between local coal seam and adjacent seam in goaf of group coal seams, the stable isotope technology was proposed to analyze the source and proportion of mixed gas in contiguous seams. Taking Tunlan Coal Mine as the research object, the carbon and hydrogen isotopes in methane and carbon isotopes in ethane of desorption gas were tested from the current coal seam and the upper and lower adjacent coal seams, and the distribution characteristics of each value were analyzed; the carbon and hydrogen isotopes were tested of mixed gas from the goaf roof fracture zone including high drainage roadway and roof borehole and the upper corner near the working face when the working face was pushed to different positions. The mathematical model of three-terminal mixed gas source identification was established to calculate the gas source proportion of each mixed gas. The research results show that: there are differences in the carbon and hydrogen isotopes median values of desorption alkanes in the upper group of Tunlan Coal Mine, in which the carbon isotope difference of methane and ethane is larger, and the difference of hydrogen isotope median value is smaller, so it could not be used as an end member gas in this test; the ratio of gas influx of each coal seam in goaf roof fracture zone and upper corner of 12505 fully mechanized mining face shows a dynamic change law, the gas influx can be divided into coal seam No.02 influx stage, coal seam No.2 influx stage and stable influx stage. In the stable gas influx stage in the roof fracture zone, the average proportion of mixed gas from coal seam No.02, coal seam No.2 and No.4 is 60%, 30% and 10%, respectively. In the stable influx stage in upper corner and buried pipeline, the average proportion of mixed gas from coal seam No.02, coal seam No.2 and No.4 is 13%, 50% and 37%, respectively. Compared with the traditional method of separate source prediction, the method of stable isotope source separation prediction can dynamically analyze the gas sources in different locations during mining, which can provide a more effective basis for accurate gas control.