The coagulation behavior of aluminum salts in a eutrophic source water was investigated from the viewpoint of Al(III) hydrolysis species transformation. Particular emphasis was paid to the coagulation effect of Al13 species on removing particles and organic matter. The coagulation behavior of Al coagulants with different basicities was examined through jar tests and hydrolyzed Al(III) speciation distribution characterization in the coagulation process. The results showed that the coagulation efficiency of Al coagulants positively correlated with the content of Al13 in the coagulation process ratherthan in the initial coagulants. Aluminum chloride (AICl3) was more effective than polyaluminum chloride (PACI) in removing turbidity and dissolved organic matter in eutrophic water because AlCl3 could not only generate Al13 species but also function as a pH control agent in the coagulation process. The solidstate 27Al NMR spectra revealed that the precipitates formed from AlCl3 and PACl were significantly different and proved that the preformed Al13 polymer was more stable than the in situ formed one during the coagulation process. Through regulating Al speciation, pH control could improve the coagulation process especially in DOC removal, and AlCl3 benefited most from pH control.