Based on mechanism of eutrophication, the cause of eutrophic formation in water is analyzed in this paper. The analysis is made from the level of nutrients (nitrogen and phosphorus), content of iron and silicon, illumination intensity, temperature, water dynamics (stream of water and wind power) and period of renewal. The purpose of the analysis is to provide a reference to keep a balance of ecology in water, control the water pollution and prevent eutrophication of water.