<p>Epoxy resin, as a crucial thermosetting polymer, is widely used in aerospace, electronics, electrical engineering, and renewable energy due to its exceptional mechanical properties, corrosion resistance, and electrical insulation. However, its inherent flammability and the environmental and health risks of some traditional halogen-containing flame retardants limit its application in safety-sensitive scenarios. This review systematically examines recent advances in halogen-free flame retardant modification of epoxy resins, focusing on material design strategies, nanocomposite technologies, and synergistic flame retardant mechanisms. Research demonstrates that phosphorus-based flame retardants (e.g., DOPO derivatives) significantly enhance flame resistance through gas-phase radical quenching and condensed-phase char formation. Multi-element synergistic systems (e.g., phosphorus–nitrogen, phosphorus–silicon) optimize flame retardant efficiency via synergistic effects, while bio-based flame retardants balance environmental compatibility and sustainability. Nanocomposite technologies incorporating materials like carbon nanotubes and graphene improve char layer stability and mechanical performance. Current challenges include the adverse impact of flame retardant loading on mechanical properties, nanomaterial dispersion stability during large-scale production, and cost control. Future research should prioritize intelligent design (e.g., dynamic crosslinking network response mechanisms), multifunctional integration (flame retardancy–thermal conductivity–smoke suppression synergy), and green manufacturing technologies to expand epoxy resin applications in electronics encapsulation and renewable energy. This review provides theoretical support and technical guidance for advancing halogen-free flame retardant epoxy resins toward high efficiency, environmental sustainability, and multifunctionality.</p>