Abstract: Some key gulfs in China are facing eutrophication related to terrigenous input and human activities. The weak hydrodynamics of the gulf brings about more serious challenges to the water management organizations. In recent years, many projects have been lunched in Zhejiang province to improve the water environment of gulfs. As the result, water qualities are significantly improved. Sanmen Bay, the second largest in the province has been a key research object of nutrition survey for almost 30 years. Based on the monitoring data in the past 30 years and the joint survey results conducted in September 2019, acquired are 54 samples from river basins, 30 samples from sea water and 6 samples from precipitation. All the samples are analyzed for nutrient contents, such as NO₃⁻ and NO₂⁻, NH₄⁺, PO₄³⁻. Based on the results and the spatial distribution patterns of the nutrients in the bay, the influence factors on the nutrient spatial distribution patterns are revealed, especially the long-term changes of nutrients and their response to human activities. At present, the eutrophication problem remains prominent in Sanmen Bay. The contents of DIN and DIP in the sea sample range from 0 to 439 μg/L and from 18 to 59 μg/L, with an average of 233 μg/L and 37 μg/L, respectively. The contents of DIN and DIP in the river sample range from 77 to 1 586 μg/L and from 3 to 126 μg/L, with an average of 466 μg/L and 48 μg/L, respectively. The contents of nitrogen and phosphorus increased from the upstream to the downstream. Agriculture has great influences on NH₄⁺-N and NO₃⁻-N, while urban living and industrial production have great influences on NH₄⁺-N, NO₂⁻-N and DIP. The high values of nitrogen and phosphorus in the bay were mainly distributed at the head of the bay and the entrance to the sea where water exchanges are rather weak. The nutrient concentration shows a decreasing trend from the high values nearshore to the low value offshore. The mariculture was the main contributor of nutrients to the environment nearshore. During the period of 1987-2007, Sanmen Bay suffered a continuous increase in water nutrients. After 2010, with the enforcement of marine and land ecological environmental management policies, DIN and DIP have showed an obvious decreasing trend, and the water environment of the bay is greatly improved.