Abstract: In order to obtain optimum PSAS (Polysilicate-aluminium sulfate) preparation conditions for removing HABorganisms, the 3 factor and 3 level orthogonal test was devised. Concentration of SiO₂, Al³⁺/SiO₂ molar ratio and pHwere chosen as the three chief factors in PSASpreparation. The experimental results showed that the optimum preparation condition for various HABorganisms was different, and according to practical experimental operation, the appropriate preparation condition was considered as: concentration of SiO₂=2%, molar ratio of Al³⁺/SiO₂=1,and pH=4.In addition, the coagulation efficiencies of PSASon various HABorganisms were studied and compared with classical inorganic flocculant AS (Aluminium sulfate). The results indicated that the removal rates of PSAS were much higher than those of AS, and the dosages of PSASwere 30～40% lower than those of ASwhen they achieved the same removal rates. On the other hand, the anti coagulation ability of HABorganisms varied with algal species, which was related to different physiological and ecological features of various algal cells.