For several reasons, the north area of the Dianchi Lake is prolific of blue-green alga. In this paper, the dynamic biomass of alga was investigated. Both of the concentrations of N and P in original water and in filtered water were also be monitored. The aim of the study is to figure out the alga's influence on the concentrations of N and P in the water, and to analyze the relationship between them by using the method of ecological stoichiometry. It is disclosed that from April to November is the main stage of alga blooming. Especially, alga blooms seriously and is uncontrolled from May to September. By data analysis, we find that the concentrations of N and P vary conforming to the dynamic of the biomass of alga, and there is positive correlation-ship between them. The correlation coefficient between the concentrations of N in water and alga's biomass is 0.955, between the concentrations of P in water and alga's biomass is 0.952. By taken the method of stoichiometry analysis, we find both the N∶P ratios of alga and water are not constant, and both of them have no significant ecological stoichiometry of the N∶P ratio. We also made correlation-ship analysis between the N∶P ratio of water and biomass of alga. The correlation-ship between them is 0.308, which illustrats that the biomass of alga is not related to the N∶P ratio of water. Thus we can declare that there is only one most efficient way to control blue-green alga booming, to reduce the concentration of N and P in water of eutrophic lakes.