Through the polycondensation of a novel thio-containing diamine, 4-amino-4′-[4-aminothiophenyl]benzophenone (AATBP), toluene-2,4-diisocyanate, and N-methyldiethanolamine (MDEA), we obtained a polymeric photoinitiator containing in-chain benzophenone, a thio group, and a tertiary amine in the same macromolecule (PUPIA). The ultraviolet–visible spectra indicated that PUPIA possessed maximal absorption similar to that of its low-molecular-weight counterpart (AATBP). Electron spin resonance analyses showed that PUPIA could generate free radicals efficiently. Differential scanning photocalorimetry studies indicated that PUPIA could efficiently initiate the photopolymerization of poly(propylene glycol)diacrylate and trimethylolpropane triacrylate. However, the photoinitiation systems of its low-molecular-weight counterparts (AATBP/MDEA or AATBP/PUPA) could hardly initiate the photopolymerization.