Dissolved oxygen is an indispensable element for the survival of marine organisms. With the increase of human activities, the situation of hypoxia in the global coastal waters has become exacerbated, which is considered as an important factor threatening the health of the marine ecosystem. The vertical distributions of water temperature, salinity and density were investigated in the offshore area of Qinhuangdao from May to September in 2017. Additionally, the average monthly oxygen consumption rate was evaluated, and hence the mechanism of hypoxia and acidification was discussed. The results showed that the water column in the studied area was vertically homogenous in May, and no obvious difference for dissolved oxygen (DO) between the surface and bottom layers, with the value larger than 8 mg/L. From June to August, the pycnocline presented in the mid of the water column. In this period, DO and pH in bottom water gradually decreased, and reached 2−3 mg/L for DO and less than 7.8 for pH in the end of August, suggesting significant hypoxia and acidification in this area. However, DO and pH in bottom water sharply increased in September, after the pycnocline disappeared. The result indicated that hypoxia and acidification of Qinhuangdao offshore waters are seasonal. DO was significantly correlated with chlorophyll a and pH, indicating that the hypoxic and acidified processes in the offshore area of Qinhuangdao were local. The evaluated oxygen consumption rate, based on the box model, in bottom water and sediment ranged from 951 mg/(m²·d) to 1193 mg/(m²·d) (mean: 975 mg/(m²·d)) from June to August in 2017. Comprehensive analysis showed that water stratification was the prerequisite for the occurrence of hypoxia and acidification in Qinhuangdao offshore waters, and DO consumption caused by organic matter decomposition was an important reason of hypoxia and acidification in bottom water.