In order to explore the effect of reservoir thermal stratification on the vertical distribution of dissolved oxygen(DO), a monitoring platform was set up in Panjiakou Reservoir in August 2019. For monitoring purposes, YSI-EXO2 monitor and NISKIN sampler were used to collect water samples and to measure the vertical distribution of water quality indicators such as water temperature, dissolved oxygen, pH and phytoplankton in the reservoir. Then, the influencing factors of the distribution of dissolved oxygen and the formation causes responsible for the metalimnetic oxygen minimum(MOM) during the thermal stratification period were analyzed based on the measurements. The results showed that Panjiakou Reservoir has undergone significant thermal stratification in August, 2019 and the water temperature presented epilimnion-metalimnion-hypolimnion structure. The concentration of dissolved oxygen in the epilimnion was higher than 6.5 mg/L, whereas in the metalimnion it was reduced significantly with a large gradient, where MOM appeared because of the thermal stratification. The concentration of dissolved oxygen in the hypolimnion gradually recovered partly. Correlation analysis indicated that dissolved oxygen was significantly correlated with water temperature and buoyancy frequency, and the thermal stratification was the main physical factor for the formation of MOM. Dissolved oxygen was positively correlated with phytoplankton biomass and pH, which was mainly associated with photosynthesis and respiration of phytoplankton. The development of MOM was promoted by the biological and chemical factors such as the respiration of plankton, the accumulation of nutrients and the oxidative decomposition of organic matters to some extent.