A novel flame retardant containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and unsaturated bonds (DOPO-HEA) was synthesized and incorporated into epoxy acrylates (EA) in different ratios. The effects of DOPO-HEA on the thermal stability, flame retardant properties, and degradation mechanisms of EA were investigated. The flame retardent properties of the resins showed that the incorporation of DOPO-HEA into EA could improve the limiting oxygen index (LOI) values and greatly reduce the peak heat release rate (pHRR) of EA. The thermal properties of the resins indicated that the incorporation of DOPO-HEA into EA could distinctly improve the char residues. Moreover, the thermal degradation processes of the EA/DOPO-HEA resins were investigated by direct pyrolysis/mass spectrometry (DP-MS). As for the variation in the condensed phase, FTIR and Raman spectroscopies were adopted to investigate the char residues. Due to the catalyzing charring effect and the gas flame retardant effect of phosphorous, the resins exhibited significant improvements in flame retardant properties.