The invention relates to the technical field of PTA (pure terephthalic acid) materials, and discloses a preparation method of PTA with a flame-retardant function. The preparation method comprises thefollowing steps: mixing a salicylic acid aqueous solution, a hypoflorous acid aqueous solution, magnesia-alumina spinel, alkali magnesium sulfate whisker and nickel iron spinel, grinding the mixture by a high energy liquid ball grinder, performing calcination, grinding by the ball grinder, performing radio frequency plasma treatment, performing ultrasonic dispersion in an organic silicone ethanolsolution, and performing filtration, washing, drying and sieving to prepare flame retardant powder; mixing acetic acid and deionized water, adding a sodium hexametaphosphate aqueous solution and the flame retardant powder, performing beating evenly, fusing a product with CTA slurry, performing crystallization three times, and performing washing, solid-liquid separation, steam drying and hydrogenation refining to prepare PTA. A flame-retardant modifier is added to the PTA preparation process, so that PTA has a flame-retardant function, and a synthetic material is prepared from PTA, interactiontime between the modifier and a substrate is long, the fusion degree is good, the synergistic effect is sufficient, and flame retardancy and mechanical properties of the synthetic material can be significantly enhanced.