Abstract: Based on the data of pH, temperature, salinity, chlorophyll a and dissolved oxygen of surface seawater in Shandong coastal area from July to August in 2020, the distribution characteristics and influencing factors of pH in Shandong coastal area were studied. The results showed that the pH of surface seawater in Shandong coastal area in summer was 7.76～8.62, with an average of 8.12 ± 0.11. The pH of Laizhou Bay was the highest and that of Bohai Bay was the lowest. In summer, the surface pH of Bohai Bay was significantly affected by the mixing of water masses caused by runoff and the mineralization and decomposition of organic matter by microorganisms. The enhancement of photosynthesis caused by the input of terrestrial nutrients was the main factor controlling the pH in Laizhou Bay. The pH of the northern coastal area of Yantai and Weihai was significantly affected by the cold water mass of the Yellow Sea, and the equilibrium process of carbonate dissociation controlled by temperature and salinity also had an impact on pH. The pH of the South Yellow Sea was not only affected by the cold water mass of the Yellow Sea, but also affected by photosynthesis.