The concept of incorporating cross-linkable precursor as the copolymerizing monomer has been proved efficient for polyesters to achieve low flammability and anti-dripping property. Adapting this concept, in our latest paper, a new co-monomer named 5-(2,5-dioxo-3-phenyl-2,5-dihydro-1H-pyrrol-1-yl) isophthalic acid (DPDPI) was synthesized and incorporated into PET chain. Results from limiting oxygen index (LOI), UL-94 and the cone calorimetric test demonstrated that these PET-co-DPDPI copolyesters possess excellent flame retardance and anti-dripping performance. UL-94 V-0 rating could be achieved with a DPDPI content of as low as 10 mol% (PET-co-DPDPI10). Results from thermogravimetry-differential scanning calorimetry (TG-DSC) and dynamic oscillatory rheology proved the existence of the cross-linking behaviour during heat. The cross-linked polymer structures were stable at high temperature and could promote the char formation during burning. According to the results of Raman spectroscopy, X-ray photoelectron spectrometry (XPS) and pyrolysis/gas chromatography-mass spectrometry (Py/GC-MS), the cross-linking and flame-retardant mechanism were proposed for PET-co-DPDPIX, that during combustion, 2π + π cycloaddition which occurred between the phenylmaleimide groups as the cross-linking reaction. Afterward, the cross-linked networks promoted aromatization and carbonization, which resulted in self-extinguishing and anti-dripping.