A series of stable composite coagulants was prepared using polyaluminum chloride (PAC) and polydimethyldiallylammonium chloride (PDM) with different intrinsic viscosity as raw materials. The enhanced coagulation processes of summer high algae content Taihu lake raw water by using the composite coagulants were reported in this paper. Jar tests were used to study the influences of the composite ratios of PAC to PDM and the intrinsic viscosity of PDM on the turbidity removal and precipitating performance. The results showed that for the summer Taihu raw water with temperature of 28-30 degrees C and turbidity of 30-33 NTU, algae content of 2.6 x 10(7) cells/L. When 2NTU turbidity of supernatant from settlement tank was required according to the current process of potable water plant in Wuxi, the dosage of 7.00 mg/L PAC was necessary, the PAC/PDM composite coagulants with the ratios (m/m) of 20:1, 10:1, 5:1 only needed dosage ranges of 3.00-2.83 mg/L, 3.50-3.49 mg/L, 5.37-4.67 mg/L with the increasing of PDM intrinsic viscosity from 0.52 dL/g to 2.46 dL/g. And compared with PAC only used, composite coagulants could decrease the PAC dose by 57.14%-59.57%, 50%-50.14%, 23.29%-33.29% correspondingly. When residual turbidity of 1 NTU was required, the dosage of 10 mg/L were necessary for the treatment with PAC , composite coagulants with the ratios(m/m) of PAC to PDM of 20:1, 10:1, 5:1 and the different intrinsic viscocity of PDM needed dosage of 8.33-3.91 mg/L, and could decrease the PAC dose by 16.7%-60.9%. Therefore, the efficiency turbidity removal and precipitating performance of composite coagulants were remarkably, and the lower the ratios of PAC to PDM and the higher the intrinsic viscosity of PDM, the higher efficiency of turbidity removal.