Abstract: The relationship between Algae biomass and nutrient condition of the water body was studied in the fishpond ecosystem in coastal polders in Jiangsu Province. The results showed that there were 7 phyla and 127 species of algae in the fishponds. During 7 months of a year (from April to October), the average population and biomass of algae were 119.39×10 ⁶ entries/L and 90.6 mg/L, respectively, which varied dynamically. Cyanophyta, Chlorophyta and Bacillariophyta were the dominant phyla of algae in the ecosystem. The major water environment factors influencing the composition of populations were N/P ratio and total P. The results of this experiment showed an extremely significant negative correlation between algae biomass and N/P ratio.