The effect, influencing factors and mechanism of modified tannin(Tanfloc)flocculation to remove Microcystis aeruginosa were studied. At the dosage of 10 mg/L Tanfloc, a high algae removal rate of 94% can be obtained within 10 minutes, but when the dosage is further increased, the removal rate decreases slightly. Tanfloc has a wide application range of initial pH value. Under acidic and weakly alkaline conditions(pH value not greater than 9. 0), the algae removal rate exceeds 90%. Under strong alkaline conditions(pH value is not less than 10. 0), the algae removal efficiency decreases gradually. Tanfloc in high algal cell density(greater than 4. 87×10⁹ cell/L)can obtain a high algae removal rate more than 95%. The pH value of water will affect the algae removal mechanism. When the pH values are 5. 5, 8. 0 and 9. 5 respectively, electric neutralization, electrostatic cluster and bridging effect are the main influencing factors of algae removal.