Abstract: The response mechanism of water quality to natural and anthropogenic activities represents a core scientific issue for water security and sustainable watershed development,with particular relevance to the Beijing-Tianjin-Hebei(BTH) region,where severe water scarcity and water-quality-related water shortages coexist. Taking this region as the study area,this study quantified the spatiotemporal evolution of anthropogenic pollutant discharges from 1980 to 2020 using the MEANS model,and revealed the response mechanisms of water quality across different hydrological periods by integrating random forest and Shapley additive explanations(SHAP) approaches. The results show that the regional pollution structure shifted from a pattern dominated solely by industrial sources to a composite dominance of urban residential and livestock breeding sources. High-emission areas exhibited three evolutionary stages: scattered point distribution,contiguous expansion,and contraction toward core areas. Pollution discharge was strongly concentrated during the flood season,with livestock breeding and crop cultivation sources serving as the primary drivers of seasonal variation, and discharge seasonality intensified in most sub-catchments. Landscape pattern factors were important drivers of water quality variations across hydrological periods,exerting a significantly stronger influence than pollutant discharge. Based on the identified threshold effects and differences among hydrological periods, this study proposes targeted strategies for precision regulation of landscape patterns and pollutant discharge reduction,providing scientific support for water pollution control and water security management in the BTH region.