The cationic polyacrylamide flocculant P(AM-DMDAAC) is prepared by copolymerizing acrylamide (AM) and dimethyl diallyl ammonium chloride (DMDAAC) initiated by H₂O₂ under UV irradiation conditions. The structural features and thermal stability of P(AM-DMDAAC) are characterized via infrared spectroscopy, thermogravimetry/differential scanning calorimetry, and proton nuclear magnetic resonance spectroscopy. The effects of multiple factors, including monomer concentration, mass monomer ratio, illumination time, initiator H₂O₂ concentration, solution pH, illumination intensity, and urea concentration, on the intrinsic viscosity of P(AM-DMDAAC) are studied. The optimum preparation conditions are found to be 30% monomer concentration, 1:4 mass monomer ratio, 0.5‰ initiator concentration, 5.5 pH, 30 min illumination time, 1800 uW/cm² illumination intensity, and 0.5% urea concentration. Under optimal conditions, the intrinsic viscosity of P(AM-DMDAAC) reaches 18.31 dL/g, which corresponds to a molecular weight of 9.6 × 10 ⁶. Sludge dewatering experiments show that P(AM-DMDAAC) exhibits superior flocculating performance compared with other flocculants.