A novel phenylethynyl-terminated cyclotriphosphazene oligoimides (hexakis (4-phenyl acetylene imide phenoxy) cyclotriphosphazene, HPAIPC) was synthesized to solve the conflict between flame retardance and antidripping of poly(ethylene terephthalate) (PET). The PET/HPAIPC composites were prepared by direct melt compounding, and the flame-retardancy and antidripping properties of the composites were investigated. The results revealed that thermal stability, self-extinguishing ability, and inhibition of melt-dripping properties of PET/HPAIPC composites were obviously enhanced by the introduction of high-temperature auto-cross-linking cyclotriphosphazene oligomides into polyester. All results of rheological analyses, pyrolysis products, and the morphology of the char demonstrated that the HPAIPC could exhibit a greater complex viscosity and enhance the physical barrier effect retarding the flammable gases, heat flux, and flame.