Using aluminium sulphate (AS) and chitosan as the main coagulant and coagulant-aid respectively, the high algae-laden water was treated by coagulation /flocculation/sedimentation (C/F/S). When the dosage ranges of AS and chitosan were 35~45 mg/L and 0.4~0.7 mg/L respectively, the removal rates of turbidity, algae cells and TOC were all enhanced apparently compared with adding AS alone. Especially the effective removal of organic matters has a significant sense. The compound action of AS and chitosan can reduce dosage of aluminum salt coagulant for meeting the treatment requirements, which means reduction of residual aluminium in treated water. The algae flocs generated by coagulation aid of chitosan were larger and more compact and had a faster settling velocity. So the application of chitosan can bring better environmental and economic benefits in water treatment.