Abstract: A flame retardant additive with sulfone and phosphaphenanthrene groups (FRASP) was successfully synthesized for improving the flame retardancy of diglycidyl ether of bisphenol A type epoxy resins (DGEBA). The chemical structure of FRASP was characterized by attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR), nuclear magnetic resonance (NMR) and elemental analysis. FRASP was added into DGEBA, and the flame-retardant DGEBA-FRASP composite resins were prepared via thermal curing reaction between 4,4’-diaminodiphenyl methane (DDM) and the mixed DGEBA. When the mass fraction of FRASP is 9wt%, the limiting oxygen index (LOI) reaches 35.9%, and V-0 rating under the vertical combustion (UL94) test is achieved. The morphology and Laser Raman spectroscopy (LRS) analysis of char residues after UL94 test indicate that the addition of FRASP favor the generation of an intumescent, tight and high-graphitization degree char layer with porous structure inside. The thermosgravimetric analysis results reveal that FRASP changes the thermal degradation process of DGEBA-FRASP composite resins and the maximum mass loss rate decreases, which results in a higher residual char. Pyrolysis-gas chromatography/mass spectrometry analysis shows that the phosphorus fragments and nonflammable gas release during DGEBA-FRASP composite resins thermal decomposition, which can dilute the O₂ and act as free radical inhibitors in combustion. Owing to the synergistic effect of P, N and S in condensed and gas phase, the DGEBA-FRASP composite resins display improved flame-retardancy.