Abstract: Dissolved oxygen (DO) is a critical indicator of ecological health in lakes and reservoirs, serving as a pivotal metric for assessing the self-purification capacity of water bodies.In particular, the reduction of DO in drinking water sources would lead to the decline of water quality and affect the safety of drinking water.The Nizitan Reservoir in Sichuan Province, located in the upper reaches of the Changjiang River, was selected as the study area.The temporal and spatial variation characteristics and stratification characteristics of the DO concentration in the reservoir were analyzed through continuous field monitoring, and the correlation between dissolved oxygen and water quality was identified using Pearson correlation analysis.The results showed that: ① The DO concentration in the reservoir was higher in winter and spring than in summer and autumn, with a large range of variation in autumn.Influenced by thermal stratification, the vertical stratification of water quality indexes and DO had significant seasonal differences.There was obvious stratification and anoxic zones across all seasons except winter.② The analysis on the relationship between the bottom DO concentration and the trophic state of the reservoir, the thermal stratification index and the hypoxia index, as well as the surface algal density and the hypoxia index, revealed that thermal stratification was an important driving factor for the change of DO concentration in the reservoir′s vertical direction, especially at the bottom.③ The correlation analysis between DO concentration and water quality in the reservoir showed that DO concentration was positively correlated with water temperature, pH, and turbidity, and negatively correlated with ammonia nitrogen concentration.DO also had a negative correlation with total phosphorus concentration in the spring and winter, but a positive correlation in the summer and autumn, which indicated that surface DO concentration was directly affected by algal reproduction.