The intumescent flame retardant (IFR) content-gradient distribution in poly (butylene terephthalate) was prepared in Co-rotating twin-screw extruder and plate vulcanizing technology. The PBT/IFR particles were fabricated by Co-rotating twin-screw extruder. The structure of IFR content-gradient distribution in PBT was fabricated by plate vulcanizing technology through melting blended two different IFR content in the PBT/IFR particles of uniform size and the same volume. The flame retardant, mechanical properties and morphological features of IFR content-gradient distribution in PBT, which were respectively obtained by the limited oxygen index (LOI), UL-94, thermogravimetry analysis (TGA), scanning electron microscope (SEM), the polarizing microscope. LOI and UL-94 tests results showed that with a little change of particle content-gradient distribution in PBT/IFR, the flame retardant property of PBT/IFR was better. Tensile and impact tests presented that a little change of particle content-gradient distribution in PBT/IFR had no effect on its mechanical properties compare to uniform PBT/IFR composites of the same IFR content. What’s more, the mechanical properties promoted in some ways.