Abstract: In the summer of 2025, the head of the Taochacha Canal at the Danjiangkou Reservoir experienced abnormally low dissolved oxygen (DO) concentrations, significantly increasing the risk of algal blooms. To ensure the long-term safety of reservoir water quality, 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 abnormally low DO concentrations. Results indicated that during the monitoring period, DO concentrations in the study area exhibited an overall fluctuating downward trend over time. Spatially, DO concentrations decreased from the center of Danku toward the head of Taochacha Canal, with stable vertical stratification observed in the water body. Correlation analysis and mechanism discussions revealed 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 and 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.