Using dimethyldiallylammonium chloride and acrylamide as the materials and ammonium persulfate - sodium bisulfate as the initiator, the preparation technologies of copolymer PDA with cationicity of 5% via one-stage addition of the monomers and stepwise increasing temperature were researched by using the intrinsic viscosity as quantitative criterion. The effects of the monomer mass percentage, reaction temperature, Na4EDTA dosage, initiator dosage and maturation temperature on the intrinsic viscosity of product PDA were studied. The results show that intrinsic viscosity of PDA with cationicity of 5% could be up to 19.4dL/g, when the monomer mass percentage was 16%, the reaction temperature was 40℃, the ratio of Na4EDTA mass to monomer mass was 2.0?0-4, the ratio of initiator mass to monomer mass was 7.4?0-4 and the maturation temperature was 60℃. The structure of PDA was characterized by FTIR and NMR.