Abstract: As we know,Silver carp (Hypophthalmichthys molitrlx) could decrease the biomass of the blue-green algae and have effect on species composition of phytoplankton,but different environment needs different density of Silver carp to control the cyanobacterial bloom. Dianchi lake is situated at Yungui Plateau,and the environment of it is different from other lakes. So the experiment was carried out. In this study, four enclosures in the same size,with an area of 100m2 and a depth of 85cm,and sediment inside,were constructed near Dianchi Lake. And the water inside the enclosures came from the surface of Dianchi lake. Three enclosures were cultured with 100g/m3,75g/m3and 50g/m3 of biomass of Silver carp respectively,and the left one was as control.During two months,the Chla. of phytoplankton were decreasing and the SD of the water were improving in enclosures cultured with Silver carp.The enclosure with 75g/m3 of Silver carp improved the ecology well.Through putting the fish into the enclosures,the outbreaking of the cyanobacterial bloom was controlled,the dominant species of algae changed,and the biodiversity of phytoplankton increased. In order to study the effects of the Potamogeton crispus L. followed with Silver carp on the ecology of the enclosures. Continual experiment was done. After culturing the fish for two months,the enclosures which had been cultured with Silver carp were planted with Potamogeton crispus L.,and the aquatic plants in enclosure with 75g/m3 of Silver carp grew better than in others,the zooplankton appeared with large amounts,and the SD of the water increased more.