In this research, the quality of water in Sansha Bay was analyzed and evaluated from May 2009 to May 2010, with 20 sampling stations, and 8 voyages. The results showed: In terms of seasonal change, the best seawater quality of Sansha Bay appeared in spring with a poor nutrient status and the tested indices were lower than the second-grade water quality criteria. The nitrogen concentration of Sansha Bay was very low in summer, while phosphate concentration was always higher than 0.045 mg/L. The quality of water in Sansha Bay in summer was at the stage of nitrogen-limited potential eutrophication. The worst seawater quality of Sansha Bay appeared in autumn. During this period, phosphate concentration in most stations were higher than 0.06 mg/L and nitrogen concentration were higher than 0.6 mg/L in some areas, which were both much higher than those in summer. Water quality of Sansha Bay in autumn was at the stage of middling nitrogen-limited potential eutrophication. Tested indices showed that the seawater quality in winter, which was similar to that of spring, fitted the third-grade water quality criteria and was at the stage of middling nutrient level. In terms of the spatial distribution, poor water quality obviously appeared in the west region of Sandu Island because of its cage culture, and better water quality obviously appeared in the east region of Sandu Island because of its laminaria culture. Downtrend of DO could be found from bay mouth to bay top. Compared with bay mouth, lower conductivity and TSP (Total Suspended Particles) were found at bay top due to runoff from inland.