Abstract: Epoxy resin (EP) is widely used in various fields of national production and life due to its excellent chemical stability, electrical properties, adhesive properties, and mechanical strength. But its flammable properties pose a threat to people's lives and property safety, thus, the flame retardancy modification of epoxy resin has always been a research hotspot. Using 3-aminophenol, 4-nitrophthalonitrile, 4-formylphenylboronic acid, and 9, 10-dihydro-9-oxa-10-phosphazephenanthroline-10 oxide (DOPO) as raw materials, a self-crosslinking flame retardant (DBPN) was synthesized through substitution, condensation, including addition reactions, and was used for flame retardancy of solid epoxy resin. The thermal stability, flame retardancy, and flame retardancy mechanism of epoxy resin composites were systematically studied. The results show that adding 6.4wt%DBPN advance the initial thermal decomposition temperature of epoxy resin composite from 372.6℃ of neat epoxy resin (EP) to 351.5℃, which is conducive to the formation of a carbon layer barrier during the combustion process, isolating the mass and heat transfer processes. Its UL-94 vertical combustion test increased from N.R to V-2 level, with peak heat release rate (PHRR), total heat release (THR), peak smoke production rate (PSPR), and total smoke production (TSP) decreased by 34.2%, 29.5%, 20.8%, and 17.8%, respectively, compared to neat EP. The flame retardancy mechanism of epoxy resin composites was proposed through the analysis of residual carbon. This work provides innovative ideas for the preparation of new halogen-free flame retardants.