Abstract: Fly ash (FA), a bulk solid waste characterized by high silicon and aluminum content, serves as a promising flame-retardant filler. However, its practical application has been hindered by poor interfacial compatibility with epoxy resin (EP). To overcome this limitation, FA was modified via NaOH alkali treatment combined with KH550 silane coupling agent to fabricate the flame-retardant filler FA-NaOH-KH550, which was subsequently incorporated into the EP matrix. The thermal stability, flame retardancy, and char-forming behavior of EP composites incorporating FA, FA-NaOH, and FA-NaOH-KH550 were systematically investigated. The results demonstrate that all three fillers significantly enhance the fire safety performance of EP. Notably, FA-NaOH exhibits exceptional capability in reducing total heat release (THR) and total smoke production (TSP). Meanwhile, the FA-NaOH-KH550/EP composite achieves optimal overall flame-retardant performance, delivering a 46.7% reduction in peak heat release rate (pHRR), an extended time to ignition (TTI) of 81 s, the highest fire performance index (FPI), and the lowest fire growth index (FGI). Mechanistic analysis reveals that the modified FA operates primarily through a condensed-phase flame- retardant mechanism. The abundant SiO₂ and Al₂O₃ within FA form a stable inorganic framework at elevated temperatures, co-construct a dense char layer with EP pyrolysis products, suppressing the transfer of heat and flammable volatiles. This work not only proposes a novel strategy for developing efficient and eco-friendly flame- retardant EP resins, but also offers a promising route for the high-value utilization of fly ash.