Abstract: Based on the data obtained from nine cruises in Sansha bay during April to October in 2021, the spatial and seasonal variations of nitrogen and phosphorus and their potential influencing factors were determined. The concentrations of DIN, NH₄-N, NO₃-N and NO₂-N were 0.26～1.24 mg/L, 0.005～1.27 mg/L, 0.048～0.76 mg/L and 0.012～0.25 mg/L, respectively. The concentrations of TP and PO₄-P were 0.03～0.28 mg/L and 0.02～0.16 mg/L. Most of the concentrations of nitrogen and phosphorus exceeded the grade IV of sea water quality standard and reached the state of eutrophication. But the concentrations also showed great spatiotemporal variations. The relative higher mean nitrogen concentrations were observed in the Yantian port and Lumen port located at the head region of Sansha bay, whereas while the highest mean phosphorus concentration was observed at stations around cage culture region within Dongwu sea. The concentrations of nitrogen in spring and summer were higher than that in autumn, and mean concentration of phosphorus was the lowest in spring. In spring and summer, terrestrial input was the dominant source for nitrogen and phosphorus pollution in surface water of Sansha bay. In autumn, concentrations of nitrogen and phosphorus were determined by the combination of multiple processes such as endogenous release, seawater dilution, cage culture, and nutrient uptake of macroalgae or phytoplankton.