Eutrophication phenomena have become more and more serious since a lot of domestic wastewater which is not deeply disposed and fertilizer were discharged into the water bodies. Many reasons cause eutrophication. The excessive multiplication of alga is a main indication of eutrophication. This paper aims to research the effect of the content of nitrogen and phosphorous on the growth speed of eutrophication alga. It is found that total nitrogen or total phosphorous or a different ratio between nitrogen and phosphorous has an effect on the growth speed of purebred alga, two kinds of farraginous alga and natural farraginous alga, and proper N/P can accelerate the growth of alga. Meanwhile, the mechanism of how to cause eutrophication is also probed into for its prediction and prevention and control.