Abstract: Adding corn straw to the anaerobic fermentation system of coal can significantly increase the gas production of coal-to-biomethane. However, the changes in substrate and microbial metabolism accompanying this combined fermentation are rarely studied. The Scanning Electron Microscope (SEM) test was utilized to analyze the microbial adhesion characteristics of coal and corn straw, while the X-ray Photoelectron Spectroscopy (XPS) test examined the changes in surface elements of coal. The results indicated that the optimal ratios of lean coal, weakly caking coal, and coking coal to corn straw were 2∶1, 2∶1, and 3∶1, respectively, and the corresponding methane yields were 17.28 mL/g, 12.51 mL/g, and 14.88 mL/g. The promotional effect of co-fermentation of different rank coal and straw was observed in the order of lean coal > coking coal > weakly caking coal. Microbial identification revealed that Sphaerochaeta and Proteiniphilum were the dominant bacterial genera in the mixed fermentation system, comprising 35.95% and 24.36% of the population, respectively. Additionally, Methanosarcina, Methanobacterium, and Methanoculleus emerged as the dominant archaea, constituting 49.71%, 31.83%, and 9.87%, respectively. In comparison with coking coal and weakly caking coal, the bacterial and archaeal genera, along with their associated gene functions responsible for carbohydrate metabolism in the combined fermentation broth of lean coal, coking coal, and weakly caking coal, were found to be dominant. This observation aligns with the principles governing methane production. Notably, in mixed substrate fermentation, as coal rank increased, bacterial presence on the coal surface gradually declined, while a substantial bacterial population consistently adhered to the surface of corn straw, indicating the latter's sustained advantage in degradation. After co-fermentation, the relative content of C element on the surface of coking coal and weakly caking coal decreased more significantly than that of lean coal, while the total relative content of oxygen-containing organic carbon ( C—O, C=O and O—C=O ) increased significantly, indicating that co-fermentation can promote the growth and metabolism of bacteria, so that more organic carbon can be converted into biological methane. The research results identified the effect of co-fermentation of different rank coal and corn straw at the micro level, which has reference significance for revealing the internal mechanism of co-fermentation of coal and corn straw.