The synthesis and thermal properties of thermoplastic poly(urethane-imide) (PUI) resins were studied. Model reaction studies on the reactions of 4,4'-diphenylcarbamatodiphenylmethane and 4,4'-diisocyanatodiphenylmethane with phthalic anhydride were performed. We found that the reaction of anhydrides with urethane groups could take place under certain reaction conditions. According to the model reaction studies, N-2-methylpyrrolidone was employed as a solvent, and no catalyst was used in the polymerization. To restrain the side reaction of anhydrides with urethane groups, we adopted a two-step chain-extending procedure in a chain-extending reaction. The inherent viscosity of PUI was 0.83-0.99 dL/g. The prepared polymers not only exhibited improved solubility in organic solvents but also formed flexible films. Thermogravimetric analysis showed that PUI exhibited a two-step thermal weight-loss pattern. The first step of the thermal degradation of PUI was attributed to the thermooxidizing cleavage of weak and labile linkage, such as urethane groups, isopropylidene, and methylene, except for imide rings. The polymer inherent viscosity decreased sharply during the first step of thermal degradation.