Abstract: A total of 32 research cruises were set to monitor the water quality parameters in the Lianzhou bay in dry, wet, and normal season between 2005 and 2015. We analyzed the changes of factors related to the eutrophication, and applied an index to show the development of eutrophication in this important bay. Finally, the relationship between the eutrophication index and the response of phytoplankton was discussed. Our results indicated that the nutrients and the maximum concentration of COD generally occurred in the wet season. The DIN and PO₄-P concentrations were higher in dry season than normal season, while the SiO₃-Si and COD concentrations were comparable between dry and normal season. The PO₄-P concentrations varied significantly in different years and exhibited an obvious increasing pattern. In contrast, SiO₃-Si concentrations decreased and there were not much changes in DIN and COD concentrations. The PO₄-P、DIN and COD concentration generally increased in dry season. It was found that the input of runoff may have a major impact on the changes of DIN and COD levels, while the sewage discharge and aquaculture industry may remarkably affect the PO₄-P distribution. The annual average eutrophication index ranged between 0.10~1.85, which was mainly attributed to the changes of DIN、PO₄-P. Our results indicated that P was limited in the Lianzhou bay. The ratio of N/P and Si/P kept decreasing in the recent 10 years, indicating the abatement of P limitation. Under the high levels of DIN and SiO₃-Si, the sharp input of PO₄-P had changed the nutrient structure, which was the major factor triggering red tides. With increasing input of nutrients into the bay, the limitation of P keep relieving and the eutrophication is becoming serious, which will eventually lead to higher ecological risk.