Poly(ethylene terephthalate) (PET) fiber with excellent flame retardant property was prepared with introducing a containing phosphorus flame retardant 10-(2’,5’-dihydroxyphenyl) -9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (ODOPB) into PET by melt blending method. The intrinsic viscosity of the modified PET was decreased after melt blending, indicated that ODOPB could promote the degradation of PET. The addition of ODOPB can increase the amount of carbon residue of PET, which can effectively reduce the heat transfer. The movement and regularity of PET molecular chain are affected by ODOPB, resulting in the reducing of the crystallization of PET. The Raman curves indicate that the addition of ODOPB can improve the regularity of carbon layer, which is conducive to achieve the effect of flame retardant. When the mass fraction of P is 0.7 %, the limiting oxygen index of sample reaches 32.4% and UL-94 vertical reaches V-2, the fiber fracture strength is 2.6 cN/dtex, which has excellent flame retardant and mechanical properties.