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A novel reactive phosphorus-and sulfur-containing flame-retardant monomer [di(allyloxybisphenol sulfone) phenoxy phosphonate, DASPP] was successfully synthesized and well characterized. Various amounts of DASPP were incorporated into the unsaturated polyester by radical bulk polymerization. The thermal properties and flammability of the flameretardant unsaturated polyester resin (FR-UPR) samples were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), limiting oxygen index (LOI) measurements, and cone calorimetry. The results showed that the introduction of DASPP into unsaturated polyester resin (UPR) can substantially improve its fire resistance and hightemperature stability. Interestingly, a linear increase in the glass transition temperature (T g ) with increasing incorporated DASPP content was observed by DSC. Scanning electron microscopy (SEM) and Raman spectroscopy studies revealed that DASPP can effectively improve the microstructure of UPR char residue and increase its graphitization degree, which can enhance UPR's thermo-oxidative stability and char yield in high-temperature regions. Furthermore, real-time Fourier transform infrared (RTIR) spectroscopy was employed to study the thermo-oxidative degradation reactions of different UPR samples, providing insight into the combustion mechanism. In addition, results from tensile testing demonstrated the improved mechanical properties for the samples containing DASPP.