Abstract: In the summer of 2025, the water body area at the Taocha Canal Headworks of the Danjiangkou Reservoir exhibited abnormally low dissolved oxygen (DO) concentrations. To ensure stable compliance of water quality at the Taocha Canal Headworks, this study systematically analyzed the patterns of DO concentration changes during the monitoring period, investigated the evolution characteristics of environmental conditions, and revealed the causes of low DO concentrations. Results indicated that: ① DO concentrations in the study area exhibited an overall fluctuating downward trend over time. Spatially, DO concentrations decreased from the center of Danjiang sub-reservoir toward the Taocha Canal Headworks, with stable vertical stratification observed in the water body. ② Correlation analysis and mechanism investigation indicated that the decline in DO concentrations resulted from the combined effects of hydrodynamics, physicochemical properties of the water body, and engineering measures. Weak thermal stratification coupled with low flow velocities inhibited vertical oxygen transport and atmospheric reoxygenation processes. Nutrients accelerated DO depletion by driving microbial aerobic decomposition, while the deployment of algal barriers further exacerbated DO decline and vertical stratification by altering local hydrodynamics and intensifying water stratification. This study recommends that future water quality management should holistically consider the ecological effects of hydrodynamic optimization, control of external nutrient inputs, and engineering interventions such as algae-blocking nets to ensure reservoir water quality safety.