A cationic poly(methyl iodide quaternized styrene-N,N-dimethylaminopropylmaleamidic acid) copolymer was synthesized through amidoacidification reaction of styrene-maleic anhydride copolymer with N,N-dimethylaminopropylamine (ring-opening reaction). Its properties in various aqueous salt solutions and pH solutions were studied by measurements of reduced viscosity and intrinsic viscosity. The results indicated that the reduced viscosity and intrinsic viscosity of this cationic polyelectrolyte were related to the type and concentration of the added salts and the results also showed a contrary tendency in some salts with monovalent acid groups to polyelectrolyte. At the same time, some salt ions were observed to strongly attract the quaternary ammonium group of the cationic polymeric side chain and resulted in agglomeration of the polymers.