The invention relates to a method for preparing a flame-retarding PPT polyester, comprising the following steps of (1) mixing 1,3-propanediol with terephthalic acid and carrying out esterification reaction; (2) adding a mixed slurry of 1,3-propanediol and phosphorus-containing copolymerization flame retardant or 1, 3-propylene glycol blending solution of the phosphorus-containing copolymerizationflame retardant to carry out the esterifiction reaction, thus obtaining a second esterification outcome; and (3) carrying out a condensation polymerization of the second esterification outcome, thus obtaining the flame-retarding PTT polyester, wherein the phosphorus-containing copolymerization flame retardant has a dual-reaction functional group and the phosphorus occupies the proportion of 1.2%-8.5wt% in the polyester; after step (2) and before the condensation polymerization in step (3) is carried out, an alkyl diol blending solution of inorganic nano-material is added in the condensation polymerization system; and the adding amount of the inorganic nano-material is 0.1%-5.0wt% of that of the PTT polyester. The PTT polyester obtained by the method has the advantages of excellent and durable flame-retarding performance, high rupture strength and excellent fiber-forming performance, thermal stability and thermo-oxidative stability performance.