Cationic Polyacrylamide P(AM-DAC-BA) was synthesized by UV initiation, with acrylamide (AM), acryloyloxyethyl trimethyl ammonium chloride (DAC), butyl acrylate (BA) as the monomers. P(AM-DAC-BA). UV spectroscopy and infrared spectroscopy were employed to study the structural characteristics. Attributions of typical infrared vibrational frequencies in AM/DAC/BA/P(AM-DAC-BA) were analysed. By comparing with infrared spectroscopy of the monomers, symmetrical characteristic of P(AM-DAC-BA) increasesd, and the infrared spectroscopy of polymerization product was simpler. The intrinsic viscosity increased with the increase in light intensity, BA content, photoinitiator concentration and illumination time. The groups of —CONH2, —COOCH2(CO), —COOCH2—(C—O—C), —CH2—N(CH3)3 group in AM, DAC, BA were selected as characteristic absorption peaks for studying. With the increase in light intensity and BA content, the characteristic peak areas increased. With the increase in photoinitiator concentration, the characteristic peak areas decreased. The characteristic peak areas decreased firstly and then increased with increasing the illumination time. But the corresponding characteristic IR absorption peaks of P(AM-DAC-BA) were similar, and the positions of characteristic peaks were basically the same.