Environmental governance transition shaped China’s five-decade water pollutant reduction

China has experienced a complex decoupling of water pollutant discharges from economic growth, shaped by a range of environmental governance interventions. However, the intervention mechanisms underlying long-term pollutant reductions remain insufficiently quantified. We proposed a long-term annual estimation model for estimating Chemical Oxygen Demand (COD) and Ammonia Nitrogen (NH 3 -N) discharges that incorporates logistic change simulation, Logarithmic Mean Divisia Index (LMDI) decomposition, and text-mining-based quantification of mandatory policy intensity to represent time-varying environmental governance. We estimated annual pollutant discharges for all prefecture-level divisions in China from 1970 to 2100 and disentangled the contributions of multiple governance measures across the generation–governance–discharge chain. Results show that pollutant discharges exceed the safe boundary in the 1990s, peak in 2006, and subsequently decline by 20%–40% by 2020. This trajectory reflects a staged governance transition from limited governance to generation control and then to end-of-pipe treatment, with respective contributions of 0.0%–2.6%, 6.1%–21.3%, and 17.2%–50.8%. This transition is accompanied by marked spatial heterogeneity. Coastal regions achieve decoupling primarily through end-of-pipe treatment (55.3% and 62.9% of COD and NH 3 -N reductions, respectively), whereas the northwestern region relies more heavily on generation control (46.9% and 28.8%, respectively). Future COD reductions are projected to be increasingly constrained by livestock-related discharges, whereas NH 3 -N discharges are projected to fall below the safe boundary around 2035, driven by continued improvements in wastewater infrastructure. Our findings reveal distinct stages in China’s environmental governance transition and provide insights for water environmental management in China and other late-industrializing countries.