Abstract: To learn the long-term variations of the eutrophication in Xiamen Sea area, we investigated the spatial and temporal characteristics of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and DIN/DIP (N/P) molar ratio based on three cruises conducted in Xiamen sea area in autumn of 2003, 2013 and 2020, respectively. We also studied the characteristics of the eutrophications in Xiamen Sea area using a third-generation potential eutrophication assessment model (PE3). Results showed that the nutrient concentrations and N/P molar ratios generally showed a decreasing trend from inside to outside of the Sea area, with higher values mainly in the inner bay and the low values outside the bay, indicating that the Xiamen Sea area was influenced by the coastal anthropogenic activities and terrestrial input. The average DIN concentrations in Xiamen sea area were 0.789±0.527, 0.539±0.234 and 0.345±0.167 mg/L, in order of 2013, 2020 and 2003, respectively. The average DIP concentrations were 0.070± 0.082, 0.040±0.021 and 0.032±0.009 mg/L, in order of 2013, 2003 and 2020, respectively. The average N/P molar ratios were 36.2± 9.0, 28.1±12.7 and 23.0±19.0 in order of 2020, 2013 and 2003, respectively. Not only is the ecological environment of the Xiamen Sea area affected by coastal anthropogenic activities and terrestrial input, but also by the surrounding mariculture activities and its water exchange. In recent years, the government has continuously strengthened the control of marine spatial planning, constructed a land-sea integrated pollution prevention system and carried out comprehensive restoration of marine ecosystem. Comparatively, the eutrophications of Xiamen Sea area in 2020 had been improved, indicating that the ecological restoration of the waters has achieved successful results. In addition, this study can provide data and theoretical support for the further maintenance and management of the ecological environment in Xiamen sea area in the future.