The influences of inorganic salts on viscosity behavior and coagulation performance of composite coagulants Polyaluminum Chloride (PAC)/ Polydimethyldiallylammonium Chloride (PDM) have been researched in this paper. Furthermore, the correlation of these two influences has been deduced. Ubbelohde viscometer was used to study the influence of inorganic salts on reduced viscosity of PAC/PDM; meanwhile jar test was used to study the influence of inorganic salts on coagulation performance of PAC/PDM. The results showed that different inorganic salts have different influences on the viscosity behavior of compose coagulants and the sequence of the influence from strength to weakness was: AlCl3 > MgCl2> NaCl, and white precipitation was formed while Na2SO4 and Na3PO4 were added in the composite coagulant solutions. The jar test results indicated that, as coagulants, AlCl3 could enhance the coagulation performance of PAC/PDM obviously, NaCl and MgCl2 had no influence on the coagulation performance, and Na2SO4 weakened the performance while Na3PO4 first weakened then enhanced it. Therefore, there is no direct correlation between the influences of inorganic salts on the viscosity behavior and the coagulation performance of PAC/PDM.