Abstract: To address issues of excessive monitoring indicators, high evaluation costs, and significant water quality heterogeneity between inflow-rivers and reservoir zones in Guanting Reservoir, in this study a cooperative water environment evaluation system for inflow-rivers and reservoir zones was established via “zoning differentiated modeling and cooperative comparative analysis”． Water Quality Index-minimum (WQI_min) evaluation models adapted to the reservoir zone and inflow-rivers were developed using full subset regression approach based on monthly monitoring data from 2018 to 2022, to systematically reveal temporal and spatial evolution patterns of water quality and their driving mechanisms． Optimal model for the reservoir zone included five variables (fluoride, chemical oxygen demand, ammonia nitrogen, five-day biochemical oxygen demand, and total nitrogen) with an adjusted R² of 0.803． The inflow-rivers model incorporating five parameters (electrical conductivity, ammonia nitrogen, dissolved oxygen, five-day biochemical oxygen demand, and total nitrogen) achieved an adjusted R² of 0.841． Multicollinearity diagnosis and Bland-Altman consistency analysis demonstrated good agreement between WQI_min and conventional WQI (consistency ratio >92%) with robust statistical structure, with WQI_min being more sensitive to water quality variations． The developed models could reduce monitoring indicators by 58% while maintaining evaluation accuracy． From 2018 to 2022, the reservoir-zone water-quality showed an overall improvement trend annually, with mean WQI_min ranging from moderate to good levels． Water quality peaked in spring (March) and reached the lowest in autumn (September)． Inflow-rivers maintained moderate water-quality levels, where flood season dilution effects improved water quality． The Xiahuayuanqiao (Yanghe River) section exhibited poor stability with higher risks of water-quality degradation during flood seasons due to non-point source pollution． WQI_min in the reservoir zone was significantly higher than in inflow-rivers, indicating the reservoir’s self-purification and dilution capacity for nitrogen pollutants． The cooperative evaluation system and WQI_min models constructed in this study can provide methodological support for efficient water-quality monitoring and precise management of Guanting Reservoir or similar large-reservoirs in semi-humid regions of northern China．