A method to purify eutrophication water body mainly purifies the eutrophication water body through the combination technics of micro-bubble air flotation and biological contact oxidation. The present invention is characterized in that micro-bubbles with a diameter in the range of 80 nm to 500Mu m which can stay 2 to 10 minutes in the water body are injected in more closed water bodies like lakes and pools and an organism reactor provided with helix fillers is arranged at the exits of the micro-bubbles. As the organism reactor contains a mixed mycelium of nitrosomona, nitrosobacteria and denitrifying bacteria, the present invention can not only play a role of decontamination and denitriding, the micro-bubbles can be able to be carriers of microorganisms that can lead the mixed mycelium to float to the surface. Simultaneously, the present invention takes a floccule formed of algaes, suspension corpuscles and water-solubility pollutants adhered on the micro-bubbles as the nutrient sourceto degrade the pollutants in water, thereby reaching the goal of purifying the water body with high efficiency.