In this test, we used phytoplankton assay procedure to carry out determinations of phytoplankton growth potential and limiting nutrients in Gengjing reservoir.The test results revealed that, in Gengying reservoir waters, the phytoplankton growth potential of the autoclaved-filtrates and the filtrates are from 0.2 to 2.1mg/L and from 0 to 1.0 mg/L, and in waterway waters, those of the autoclaved-filtrates and the filtrates are from 0 to 3.3 mg/L and from 0 to 2.2 mg/L.The phytoplankton growth potential of the autoclaved filtrates is approximately two times higher than those of the filtrates.When phosphorus is adjusted beyond 0.5 mg/L, the growth of the phytoplankton is especially very high.Moreover, the limiting concentration of the phosphorus for the growth of phytoplankton is in the range of 0.025 to 0.05 mg/L; the phytoplankton growth potential of the grit chamber sediment samples is higher than those of the waterway sediment.