A simple and green method for the preparation of solid polymeric bisphenol A bis(diphenyl phosphate) (PBDP), aimed at improving the flame retardancy of epoxy resins (EP) is presented. The PBDP was synthesized through melt polycondensation and has been extensively characterized by FTIR, ¹H NMR, and ³¹P NMR. Flame retardant properties as well as thermal properties of epoxy resin composites have been investigated by various methods. The incorporation of PBDP into EP led to a superior flame retardant performance, such as reduced heat release rate, and higher char yield during combustion, compared to pure EP. The UL-94 V-0 rating was achieved at 20 wt% of PBDP in the composite and the glass transition temperature (T_g) was maintained at the high level of the pure EP. Meanwhile, a slight improvement in fracture toughness was observed in the EP/PBDP composites compared to pure EP. The flame retardant mechanism of PBDP combines condensed phase and gas phase mechanisms such as the catalysis effect, char-forming effect, gas activity, and the improved thermal stability of char layer. Moreover, the synthesized PBDP provides a novel flame retardant for EPs that imparts them with high T_g, high fracture toughness, and excellent flame retardancy.