Abstract: The research area is Fukang Mining Area in the southern margin of Junggar Basin. The gas composition, methane carbon isotope, methane carbon isotope and carbon isotope of CO₂ were tested in 10 CBM wells in the research area, and combined with the special geological environment characteristics of the area to discriminate the origin of CBM. It is found that the CBM components in Fukang Mining Area include CH₄（81.79％), CO₂（14.36％）, N₂（2.28％） and C₂+（0.99％）. Among them, the distribution range of methane carbon isotope （δ¹³Ｃ₁） is -58‰ to -49‰ with an average value of -53‰; the distribution range of ethane carbon isotope value （δ¹³Ｃ₂） is -32.2‰ to -23.1‰ with an average value of -28.1‰; and the distribution range of carbon isotope value δ¹³C（CO₂） of carbon dioxide is +8.5‰ to +14.2‰ with an average value of +12.9‰. The CBM in the study area is of mixed origin gas, among which the thermogenic gas is of thermal degradation origin, and the secondary biogas is mainly the product of acetic acid fermentation. The steep coal seam and burning area provide a channel for microorganism to enter the coal seam. The flow direction of groundwater is opposite to that of CBM, and which has the function of hydraulic blocking and gas control. The groundwater environment with suitable temperature, slightly acidic and low salinity provide the living environment for the microorganism. The stagnant water layer in the burning area and the roof and floor of the coal seam effectively trap the CBM reservoir to ensure the storage of the mixed gas.