The mining of medium- to high‑sulfur coal in karst areas has led to serious acidification problems in surface water, thus encouraging a re-evaluation of DIC transformation and CO₂ source-sink relationships in karst watersheds. The weathering of limestone and sulfide-rich coal measures jointly influence the pH of the Huatan River in karst areas in Southwest China, which is lower in the rainy season and higher in the dry season. Due to CO₂ degassing, DIC concentration tends to decrease along the flow direction, while δ¹³C-DIC gradually becomes heavier. In general, DIC transformation in the Huatan River is controlled by AMD input, CO₂ degassing, organic matter (OM) degradation, and the dissolution and precipitation balance of carbonate minerals in different seasons. In spring, the mineralization of OM from terrestrial and domestic sewage gradually enhances and replenishes DIC in the water. As the pH increases in this season, the capacity for buffering CO₂ increases. Meanwhile, OM degradation generates a large amount of CO₂ in summer, and carbonic acid begins to dissolve limestone. In autumn, the pH decreases due to the enhanced weathering of sulfide-rich coal measures and the mass input of AMD. Thus, the river shows the ability to drive CO₂ outgassing. In winter, CO₂ degassing gradually weakens, DIC concentration is at its lowest, and δ¹³C-DIC reaches the heaviest value.