The new ternary cationic flocculant P(AM-DMC-DMDAAC) was synthesized by aqueous solution polymerization with raw meterials of acrylamide (AM) , methacryloyloxyethyl trimethyl ammonium chloride (DMC) and dimethyl diallyl ammonium chloride (DMDAAC). Effects of Cationic monomer ratio, monomer concentration, reaction time, amount of initiator and reaction temperature on the molecular weight of the products were investigated. Results indicate that the molecular weight of copolymer is up to 5.1?06, and has excellent water-dissolubility. At the same time, the terpolrmer shows high capability of flocculating performance in slurry.