Toxic effects of L7 lyophilized powder of extracellular active components (L7-LPEAC), extracted from the Algae-lysing bacteria L7, on Chlorella pyrenoidosa were studied according to the changes of effective photosynthesis rate (EPR), the protein content, the chlorophyll a content and the MDA content of algae. The results showed that the growth of alga was promoted at low concentrations of L7-LPEAC (0.80 g/L, 1.25 g/L). The 96 h-EC50 and 120 h-EC50 upon Chlorella pyrenoidosa are 5.75 g/L and 2.55 g/L, respectively. The chlorophyll a content increased firstly and then decreased at high concentrations of L7-LPEAC (≥2 g/L), so did the protein content. Compared with the control group, there is a significant statistics difference (P<0.05) in the treatment groups (2.00 g/L, 3.13 g/L, 4.90 g/L), while extreme significant statistics difference (P<0.01) in the other treatment groups (7.67 g/L, 12.00 g/L) after 72 h. The inhibition rates of chlorophyll a content of Chlorella pyrenoidosa in all treatment groups reached over 60% after 120 h. For the remediation of eutrophic nature water body, it can not only kill the algae which caused the water eutrophication, but also maintain the balance of the aquatic ecosystem well by adding some proper concentrations of L7-LPEAC.