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A poly (cyclotriphosphazene‐resveratrol) (PRS) microsphere was synthesized via a feasible in‐situ template approach and applied to poly (ethylene terephthalate) (PET) for ameliorating its flammable and melt‐dropping issues. The prepared PRS microspheres displayed superior thermal stability and flame retardancy. By incorporating only 2 wt% of PRS microspheres into PET matrix, the limiting oxygen index value was increased from 23.3% to 28.3% and reached the UL‐94 V‐0 level without any after‐flame time. Cone calorimeter test demonstrated that the HRR and THR of PET/FR2.0 composite decreased significantly by 31.1% and 13.5%, respectively, which remarkably increased the fire safety of PET. On the other hand, the tensile strength and elongation at break of PET/FR2.0 composite decreased slightly, whereas the tensile modulus was increased by 10.5% than that of the pure PET. Moreover, the fire‐retardant mechanism of PRS microspheres was fully investigated, revealing that PET/FR system would generate compact and continuous char layers rapidly during combustion, the underlay materials would not be further cracked due to the isolation of external heat and oxygen. Nonflammable gases such as ammonia were also released to dilute the concentration of combustible gases around the PET matrix, achieving an efficient flame retardant effect. [ABSTRACT FROM AUTHOR]