The cause of dissolved oxygen (DO) evolution in deep reservoirs still remains unclarified. However, it is important to explore the evolutionary mechanism for water quality protection and management. This paper systematically analyzed the spatiotemporal variations of water temperature and DO, as well as the water quality response to these factors, in the Panjiakou Reservoir, a large deep reservoir in Beijing-Tianjin-Hebei region. Results show that the reservoir has a significant seasonal thermal stratification during mid-April to the end of November, which creates a vertically specific environment for the DO stratification. Similar to the thermal stratification, the DO stratification also shows a three-layer structure from top to down, namely, the mixed layer, the oxycline and the oxygen-deficient layer. Different biochemical processes in each layer determinate the spatial variability DO evolution. Due to excessive planktonic algae, DO is supersaturated in the mixed layer. While in the oxycline layer, DO concentration declines sharply as a result of respiration and organic matter decomposition, becoming hypoxic (i.e., DO<2 mg/L) in July and August. Due to the oxygen consumption of severely contaminated sediments in the oxygen-deficient layer, DO concentration declines continuously, potentially leading to hypolimnion at the bottom of the reservoir at the end of thermal stratification. Meanwhile, in this area, Mn reduction reaction occurs and the Mn-P is released to the hypolimnion. However, there is no Fe reduction reaction or Fe-P released. In summary, the Panjiakou Reservoir is approaching a critical point of hypoxia and massive release of endogenous pollutants. Special attentions should be paid on its water environment treatment.