Abstract: Biodegradation on coal has attracted extensive attention from scholars worldwide for its advantages of in⁃ creasing gas content， porosity， and desorption. To explore coal reservoir microscopic pore structure evolution dur⁃ ing the process of biodegradation， taking a case of the Jurassic coal in the Huangling mining area at the southern mar⁃ gin of Ordos basin， the coal pore structure in situ is compared with that of after⁃degradation using field emission scan⁃ ning electron microscopy (FE-SEM) technology. Direct evidence of biodegradation on the coal surface is discovered. Based on the image binarization and box⁃dimension fractal method， the fractal dimension variation of coal reservoir pore structure before and after biodegradation is quantitatively characterized. The results show that the pores and fis⁃ sures of raw coal samples in the Huangling mining area are well developed. The primary pores are dominated by gas holes， while the secondary pores are mainly composed of crushed， brecciated， and friction pores. The exoge⁃ nous fractures are dominated by cracks with a width of 1-2 μm. After biodegradation， the shape and size of pores in the coal reservoir are changed， and the pores diameter increases. Under the action of biodegradation， the organic matter of coal becomes loose and a large number of intergranular pores are regenerated. Crawling traces of microbial activity are found on the surface of the coal matrix， supplying the direct evidence of microbial action on coal surface. After bio⁃ degradation， the fractal dimension of the coal pore structure decreases， the coal surface becomes smooth， and the pores’structure become simple. The value of original fractal dimension is positively correlated with its variation， indica⁃ ting that the more complex the original pore structure of a coal reservoir， effect of the better the microbial modification.