Abstract: In order to improve the conversion efficiency of lignite to biomethane, the gas production effect of mixed fermentation of waste oil and lignite was discussed in this paper. NMR carbon spectroscopy, three-dimensional fluorescence and macro transcriptomics were used to analyze the internal mechanism of transformation efficiency changes. The study showed that the addition of waste oil increased the cumulative methane yield of lignite by 3.89 times, and the unit methane yield reached 31.2 mL. The percentage of aliphatic carbon in lignite decreased by 11.4%, and the content of fulvic acid like phenol hydroxyl, ketone carbonyl, carbonyl and other groups increased significantly. At the same time, the proportion of active methanogenic archaea such as Metanothrix and Metanoculleus increased by more than 50% at the peak of microbial flora. Glycoside hydrolase, fatty acyl coenzyme A dehydrogenase and other enzymes related to carbohydrate and fatty acid degradation increased significantly. With the addition of waste oil, the conversion of acetic acid and CO₂ to methane accounted for 59.2% and 40.4%, respectively. Functional enzymes such as formylmethane furan dehydrogenase and acetic acid kinase were also improved, promoting the conversion of biomethane. The addition of waste oil can significantly promote the production of biomethane from lignite, which provides a practical solution to the problem of low gas production from single lignite anaerobic fermentation. The reduction of fat carbon rate in lignite indicates that microbial flora is more likely to react with aliphatic methyl carbon, aromatic methyl carbon, methylene and methoxy in the carbon skeleton macromolecular structure of lignite in anaerobic fermentation to convert methane. The analysis results of liquid components show that the content of biodegradable organic molecules increases, which comes from the function group shedding of lignite due to the action of microorganisms and the biochemical reaction of microorganisms. The addition of waste oil can significantly change the composition of microbial community, improve its gene function expression, and promote the generation of methane.