he polluted water with abundant nourishment cause phytoplankton, such as cyanobacteria, to grow rapidly, which brings great harm to environment. In the present paper, the absorption spectrum of cyanobacteria was measured and analyzed in order to estimate the content of the chlorophyll accurately. The same amount of cyanobacteria was separately cultured in pure water and lake water for different time. The chlorophyll was extracted from the cyanobacteria for the same time by 95% of ethanol. Then the ethanol extract was tested by ultraviolet visible spectrometry. The results show that the absorption spectrum of the chlorophyll has three absorption peaks at 279.5， 436.0 and 664.5 nm respectively. However, the absorbency at 279.5 nm cannot reflect the content of the chlorophyll. The absorbencies at 436.0 and 664.5 nm have linear relationship with the content of chlorophyll. Moreover, the dispersion between the absorbency at 436.0 nm and the absorbency at 664.5 nm can reflect the content of chlorophyll more accurately. The research provides the experimental and theoretical basis for the highly accurate detection of the water quality.