Abstract: Under the multiple effects of climate change and human activities, eutrophication and the resulting seasonal hypoxia are the core of the coastal ecological security risk assessment. This paper analyzes the distribution and influencing factors of DO according to the results of field investigation in the central Bohai Sea in summer of 2020. The results showed that in the summer of 2020, a hypoxic area of about 1300 km² appeared in the bottom of the northeastern waters of the Yellow River Estuary, the center of the hypoxic area was located at 38°N, and the lowest concentration of DO was 2.18 mg/L. In the summer of 2020, the formation of the bottom hypoxic zone in the central Bohai Sea was affected by the combined effects of water stratification and eutrophication. The stratification hinders the vertical mixing of the water and reducing the replenishment of DO at the bottom; eutrophication leads to the accumulation of bottom organic matter, and the increase of bottom organic matter intensifies the decomposition process, resulting in the consumption of DO and the decrease of pH. The research on the hypoxia phenomenon in the bottom of the central Bohai Sea in the summer of 2020 can provide a reference for the ecological environment monitoring and ecological security risk assessment of the Bohai Sea in the future.