In this study, a complex UV initiation system was established by utilizing low light intensity and then high light intensity with redox initiator and azo initiator compound. Acrylamide (AM) and diallyldimethylammonium chloride (DMDAAC) were used as monomers to copolymerize P (DMDAAC-AM) (PDA). Initiators with different activation energies were injected to the system, and UV light with were different intensities is controlled to generate radicals at different times. Therefore, high intrinsic viscosity PDA could be prepared as a consequence of even and thorough polymerization. Effect of factors on the intrinsic viscosity of PDA was investigated, and the optimal synthesis condition was obtained. The results indicated that the highest intrinsic viscosity of 19.60 dL/g was reached at V-50 concentration of 0.3‰, (NH₄)₂S₂O₈-NaHSO₃ concentration of 0.4‰, low light intensity of 8.5 mW/cm² then increased to 13 mW/cm², mass ratio between AM and DMDAAC of 8:2 and total monomer mass percentage of 20%. It was confirmed that PDA was the copolymer of AM and DMDAAC through proton nuclear magnetic resonance spectroscopy and Fourier-transform infrared spectroscopy. In addition, the prepared PDA possessed an acceptable thermostability and favorable flocculation performance which can be confirmed through thermos gravimetric analysis and the flocculation experiment.