Abstract: The dark mudstone of the Suotoushan Formation in the Lower Devonian of the Yanyuan Basin represents a new stratigraphic horizon for shale gas exploration in the Upper Yangtze region. Investigating its sedimentary environment and organic matter enrichment mechanisms holds significant implications for unconventional oil and gas exploration along the western margin of the Upper Yangtze Plate. This study reconstructs the paleoclimate, paleoproductivity, paleo-redox conditions, and paleosalinity during the deposition of the Suotoushan Formation dark mudstone through analyses of rock macro/microscopic characteristics, mineral composition, elemental geochemistry, and organic geochemistry, while exploring organic matter enrichment mechanisms. Results indicate that the Suotoushan Formation was deposited in a rimmed platform environment, comprising five sedimentary subfacies: lagoon, reef flat, platform margin, collapsed slope, and basin. The mudstone exhibits favorable shale gas generation potential with an average TOC content of 1.86%, mean vitrinite reflectance (Ro) of 1.15%, dominant Type II1 and II2 organic matter, and high brittleness index averaging 64.41. During deposition, the basin experienced warm-humid paleoclimate, hydrothermal activity, high aquatic productivity, anoxic reducing bottom water, and low salinity conditions. The study concludes that the combined effects of restricted lagoon environment, warm-humid climate, anoxic conditions, low-salinity water, and elevated paleoproductivity collectively facilitated the development of organic-rich mudstone in the Suotoushan Formation.