Abstract: A simple ideal thermodynamic model of hydrolysis precipitation of alumnium(Ⅲ)inthe presence of phosphate radical was established according to the results of base titration experiments and mechanism of phosphate radical action on hydrolysis precipitation processes of alu-minium (Ⅲ).An excellent fit between the ealeulated titration curve based on the model and the experimental curve was obtained.The model reveals to a certain extent the hydrolysis precipitation processes characteristics of aluminium(Ⅲ)in aluminium (Ⅲ)-phosphate solutions;and is a theoretical base for the study of polymeric phosphato-aluminum chloride.