Abstract: Eutrophication can lead to oceanic problems, such as harmful algal bloom, hypoxia and ocean acidification. Eutrophication is also an important index to evaluate the health of coastal waters. This study analyzed the water quality parameters during 2010−2021 from Sandu bay, evaluated the changes in seawater eutrophication in Sandu bay, and accounted the pollution sources that affect water quality. Taking 2020 data as an example, the marine pollution source list was constructed, and the main factors affecting seawater quality and eutrophication were analyzed. The results showed that the eutrophication situation of Sandu bay was getting worse. The input of nitrogen and phosphorus is the main reason for the intensification of eutrophication in the bay in recent years. Overall, marine aquaculture is the main source of nitrogen and phosphorus input into the seawater of Sandu bay, with river input being a secondary source of nitrogen input and direct discharge into the sea being a secondary source of phosphorus input. Therefore, in order to improve the water quality and eutrophication status of Sandu bay, it is necessary to focus on the pollution from marine aquaculture industry.