In this study, a cationic polyacrylamide (P(AM-DAC)) was synthesized via the copolymerization of acrylamide (AM) and acryloyloxyethyl trimethyl ammonium chloride (DAC) under the initiation system of ultrasonic with VA-044. In the synthesis, the short preparation time and high preparation efficiency were found. The effects of key factors in the preparation on the intrinsic viscosity of P(AM-DAC) were investigated, and the optimal synthesis condition was obtained. The results indicated that the highest intrinsic viscosity of P(AM-DAC) was reached at the molar ratio of AM to DAC of 3:2, the initiator concentration of 0.02%, and ultrasonic time of 20 min and pH of 4. It was confirmed that P(AM-DAC) was the copolymer of AM and DAC, and the functional groups of -NH₂, C=O, -CH₂-N⁺ and -OH were found in it. P(AM-DAC) possessed the acceptable stability at the temperature range of 30 to 200 ℃.