The poly(ethylene terephalate)(PET)/hexakis [p-(hydroxymethyl)phenoxy] cyclotriphosphazene (PN6)/poly(2-phenylpropyl) methylsiloxane (PPPMS) composites were prepared, the flame-retardancy and anti-dripping properties were also investigated. The results indicated that the contents of PN6 and PPPMS had great effect on the melt behavior, crystallization and thermal properties of PET composite. The introducing of PN6 and PPPMS into PET would accelerate the char formation and anti-dripping during the flame-retardant PET combustion process. The analysis of morphology and chemical components of the charred residue displayed a synergistic effect of the PET/PN6/PPPMS system by providing a compact and intact barrier and promoting the accumulation of silicon elements in the outer surface of char residues.