In order to solve the flammable problem of poly(ethylene terephthalate) (PET), a flame retardant, 2- (6- oxido- 6H- dibenz <1,2>)oxa- phosphorin- 6yl)- 1,4- hydroxyethoxyphenylene (DOPO- HQ- HE), was synthesized by using 9,l0- dihydro- 9- oxa- 10- phospha- phenanthrene- 10- oxide (DOPO), p- benzoquinone and ethylene carbonate as raw materials. Copolyesters with good thermal and flame retardant properties were synthesized by copolymerization of terephthalic acid, ethylene glycol and this flame retardant. The structures of flame retardant and copolyesters were characterized by ¹H- NMR and FT- IR. The thermal properties, flame retardant properties and mechanical properties of copolyesters with different flame retardant contents were investigated by thermogravimetric analysis test (TGA), differential scanning calorimetry test (DSC), limiting oxygen index (LOI), UL-94, cone calorimeter test (CCT) and tensile test. The results show that the flame retardant and copolyesters were successfully synthesized. The effect on the thermal decomposition performance of copolyesters with the introduction of flame retardant is not obvious. When the flame retardant mass fraction is within 5% , copolyesters can maintain great mechanical properties. Flame retardant gives PET excellent flame retardant properties, UL-94 grade and carbon forming capability. When flame retardant mass fraction is 5%, LOI of PET-Q₅ increases from 22.8% to 32.5%, UL-94 grade meets V-0, the total heat release (THR) and total smoke rate (TSR) respectively decrease by 34.6% and 11.7%, and the residual carbon increases 4.4% at 590 ℃ compared with PET. SEM and Raman show that the number of holes on the residual carbon surface of copolyesters is reduced, and the carbon layer becomes denser. The dense carbon layer can isolate oxygen, hinder the combustion of copolyesters, reduce the release of gas and smoke, which further proves the effect of this flame retardant on the condensed phase of copolyesters.