Underground reservoirs were used to purify water through filtration, adsorption, and biodegradation methods in coal mines. However, their effects on the composition and metabolism of microbial communities in mine water remain unknown. In this study, influent and effluent samples from underground reservoirs in seven coal mining areas were analyzed to compare their microbial community compositions and metabolites. The results indicate that the underground reservoirs can significantly reduce the chemical oxygen demand (COD) levels of mine water (values range from 7.79 to 8.50 for influent and 7.40 to 8.50 for effluent) and regulate water chemistry characteristics such as pH (values range from 7-1980 for influent and 5-20 for effluent). Since COD reflects the quantity of reducing substances in water, while total dissolved solids (TDS) indicates the total amount of dissolved solids, both parameters directly influence the physicochemical properties of water. This, in turn, affects microbial growth and metabolic pathways. Consequently, changes in these factors can lead to variations in microbial community composition as well as decreases in the Chao1 and Shannon indices. Underground reservoirs decreased the relative abundance of phyla and , and the decreased microbial groups were mainly belonging to phylum . In addition, higher concentrations of metabolites, including lipids and lipid-like molecules, mixed metal/nonmetal compounds, hydrocarbon derivatives, nucleosides, nucleotides, and analogues were detected in the effluent samples, and most of them were related to taurine and hypotaurine metabolism, selenocompound metabolism, glyoxylate and dicarboxylate metabolism, riboflavin metabolism, and the citrate cycle. In summary, this study provided theoretical and experimental support for the evolution mechanism of mine water quality in coal mine underground reservoirs.