Abstract: To study the distribution characteristics of nutrient and the status, trend and causes of eutrophication in the typical mariculture bay (Qinzhou bay, south China), the nutrients concentrations and other physicochemical parameters were investigated during two cruises in February (winter) and August (summer) 2018. The concentration of dissolved inorganic nitrogen (DIN) and phosphate (PO₄³⁻) ranged from 41.1～664.8 μg/L and 3.7～40.0 μg/L, respectively. Nitrate (NO₃⁻) was found to be the dominant component of DIN (accounting for 77% of DIN), followed by ammonium (NH₄⁺) (accounting for 16% of DIN) and nitrite (NO₂⁻) (accounting for 7% of DIN). According to the analysis of the correlation and principal component of nutrients and other physicochemical factors, the input of land-based pollutants was showed to be the dominant factor affecting the distribution of nutrient during the winter, while in summer, except for the input of land-based pollutants, the impact of biological processes on nutrient can not be ignored. The seawater quality of the bay was analyzed by eutrophication index ( EI ), and the results showed that the EI values ranged from 0～19.65 (average of 4.06), and the eutrophication degree in summer was higher than that in winter, which was at the moderate eutrophication. As the significant decrease of the N/P value compared with the historical data of nearly 40 years, the eutrophication status in Qinzhou bay was increasing significantly, which was due to the increase of phosphorus discharge for the past years.