Simple,effective and safe flame retardants are required to improve flame retardant properties of polymer fibers.However,traditional additive flame retardants,such as halogen-flame retardants and intumescent flame re-tardants,are likely to cause phase separation of functional phases due to their poor dispersibility and compati-bility,or are difficult to be suitable for the high temperature processing conditions of melt-spun fibers.Here,in an effort to develop a practical flame retardant system in which zinc diphosphinate (DEPZn) and D-glucose (DG)were selectively incorporated into polyethylene terephthalate (PET) fiber was developed.As a result,the dense nano-scale zinc phosphate microspheres were formed on the surface and inside the residual carbon during combustion.Thus,PET fibers were endowed with excellent flame retardancy through a thermal barrier and enhancement of physical strength for the carbon layer.Moreover,a synergistic flame-retardant effect was found between DEPZn and DG.DG reduced the size of the zinc phosphate nanosphere from 200 nm to 50 nm,making the carbon layer denser and smoother.As a result,the peak heat release of the resultant PET composite fiber decreased to 410 kW/m2 compared 1276 kW/m2 for neat PET fiber.Moreover,the total smoke release also dropped from 71 MJ/kg of neat PET fiber to 64 MJ/kg for PET composite fibers.These results provide a promising strategy for the production of industrialized PET flame retardant fibers.