Conventional and chain-extended UV-curable waterborne polyurethane-acrylate (PUA) ionomers were prepared from diisocyanate, polyethylene glycol (PEG), dimethylolpropionic acid, and hydroxyethyl methacrylate, and identified with FTIR spectra and 500-MHz H-1-NMR spectra. The number-average molecular weight (M-n) and polydispersity of chain-extended PUA were determined by gel permeation chromatography. For the synthesis of chain-extended PUA, water was employed as the chain extender. The two kinds of PUA prepolymer could be easily dispersed in water in the form of self-emulsified latex after the carboxyl group attaching to the backbone of PUA was neutralized with tertiary amine. The effects of M-n of PEG, carboxyl content, and type of diisocyanate on the interfacial tension and rheological behavior of PUA dispersions were investigated. The chain-extended PUA prepolymer could photopolymerize to a greater extent than the conventional PUA, as indicated by differential photocalorimetry. The photopolymerization kinetics of chain-extended PUA, based on different substrates, were also investigated. The differential scanning calorimetry analysis for the photo-cured films from PUA dispersions suggested that lower M-n of PEG tended to favor phase mixing between soft and hard segment phases, and higher M-n of PEG would provoke phase separation.