In this paper, new composite coagulants, polymeric ferric aluminum-polydimethyl diallylammonium chloride, were synthesized by combing Polymeric ferric aluminum chloride (PFAC) with polydimethyl diallylammonium chloride (PDMDAAC) under different molecular weight ratio (Aluminum to PDMDAAC, Wr=10, 4, 2 and 1). Furthermore, flocculating mechanisms and performance of these new composite coagulants were investigated in treating surface water. The growth of flocs during coagulation using composite coagulants were measured under optimal dosage and neutral conditions. The results indicated that the new composite coagulants exhibited better coagulation performance than PFAC for removing turbidity and dissolved organic matter. For the composite coagulants, the predominant coagulation mechanisms were bridge formation and charge neutralization.