Abstract: Taking No. 5 coal seam in Hongyi mine field of Ningxia as the research object, this study combined channel sampling with experimental analysis, comprehensively employing coal petrology, coal quality analysis, and coal facies indices such as GI–TPI, GWI–VI, and T–D–F diagrams, to systematically investigate the evolutionary characteristics of the peat swamp environment during the coal-forming period of No. 5 coal seam and its control over coal petrology and quality. The results show that No. 5 coal seam is a medium-ash, high-volatile, medium-sulfur gas coal. The peat swamp environment underwent a three-stage evolutionary process of “extremely wet reducing → dry oxidizing → extremely wet reducing”. The vertical evolution pattern is significantly correlated with parameters such as the vitrinite/inertinite ratio (V/I), gelification index (GI), and groundwater influential index (GWI), revealing the critical control of water-cover conditions and redox environment on maceral differentiation and on sulfur content, ash yield, and volatile matter, providing a theoretical basis for guiding the clean and efficient utilization of coal.