Based on investigation data obtained from August 2012 to July 2013, the surface water temperature (SST), salinity (S), pH, transparency (SD), dissolved oxygen (DO), chemical oxygen demand (COD), suspended solids (SS), Chl.a and nutrients were analyzed in the mariculture area of Yantian Harbor, Sansha Bay. And the water quality and eutrophication conditions were also evaluated on the basis of organic pollution index (A), eutrophication index (E) and nutrient quality index (NQI) methods. The results showed that the suspended solids complied with the Grade Ⅲ, and the pH generally complied with the Grade Ⅲ to Ⅳ, while the inorganic nitrogen (DIN) and inorganic phosphorus (DIP) complied with the Grade Ⅳ or even worse than Grade Ⅳ according to "Seawater Quality Standard" (GB 3097-1997). The average concentration of DIN and DIP was 0.644 and 0.061 mg/L with the range of 0.256 -1.147 and 0.027 -0.103 mg/L, respectively. NO₃-N was the main form of DIN, which accounted for 74.83% of DIN. The DIN and DIP concentration in this mariculture area was at a higher level. The values of A, E and NQI demonstrated that the seawater was under serious eutrophication condition, and the organic pollution was also very serious. The self-pollution of mariculture was the main cause leading to serious eutrophication situation.