Effects of fish cage culture on plankton biodiversities and water quality were studied at five sampling stations(Ⅰ-Ⅴ) in the summer of Jiantiao at Sanmen Bay.29 species of plankton were found,among which 14 species were phytoplankton and 15 species were zooplankton.The dominant species of phytoplankton and zooplankton were Skeletonema costatum and Acartia pacifica,respectively.The average density of phytoplankton and phytoplankton Shannon-Weiner biodiversity index in the surface water layer were 43 328 ind·L^-1 and 0.826,respectively.The average density of zooplankton and zooplankton Shannon-Weiner biodiversity index in the whole water layer were 389 ind.m^-3 and 2.964.The result of regression analysis showed that the density of phytoplankton increased rapidly,which was caused by high concentrations of DIN,DIP in the fish cage culture region,and biodiversity index of zooplankton also increased with increase of DIN and DIP.Biodiversity index of phytoplankton had a positive linear relationship with water body E value,and there was a significant positive linear relationship(R²=0.781,p<0.05) between biodiversity index of zooplankton and water body E value.