Lake Xuanwu is a typical urhan eutrophic lake. Microcystis blcxm broke out in a large area of Lake Xuanwu in the sum-mer of 2005. The main method used to control the Microcystis bloom was the application of modified clay. In order to discover thechanges of the water ecological situation, the following tests were performed; inquiry into the changes of the quality of water andphytoplankton after the treatment, the analysis to the type and quantity of phytoplankton and physico-chemical indexes.The resultsshow; the amount of phytoplankton reduced, while the species increased after the treatment, the concentration of Chl. a and TP re-duced, with a substantial increase in SD, the concentration of TN was undulatory with phytoplankton density and Potarru}geton cris-pus growth.