In this paper,the reasons for eutrophication,the situation of algae pollution,the species,the main factors leading to the algal blooms and the negative effect of the different alga on the water treatment process were discussed.Enhancing coagulation, adsorption of activated carbon and oxidation of potassium permanganate were the three methods usually used,which inprove the removal efficiency of alga.New methods to prevent the hazards of alga are proposed based on the development of researches on water purification process.