A novel phosphorus-nitrogen-boron containing flame retardant additive of poly(phosphoric-boric acid) piperazine, defined as PPBAP was synthesized by the salt-forming reaction between anhydrous piperazine and phosphoric/boric acid, and the dehydration polycondensation under heating in nitrogen atmosphere. The synthesized PPBAP and curing agent m-phenylenediamine were blended into epoxy resins (EP) to prepare flame retardant EP thermosets. The results demonstrated that the flame retardant EP thermosets successfully passed UL-94 V-0 rating during vertical burning tests and the limited oxygen index value was as high as 31.9% by incorporating 4 wt% PPBAP. The prepared PPBAP possessed excellent charring capability, meanwhile, PPBAP stimulated epoxy resins matrix decomposed and charring ahead of time due to its catalytic decomposition effect, which led to a higher char yield at high temperature. The incorporation of small amount of PPBAP almost not affected the glass transition temperature of EP thermosets. There are good compatibility between PPBAP and EP matrix, and the EP/PPABP thermosets remained excellent mechanical performance. The introduction of PPBAP benefited to the formation of a sufficient, more compact, homogeneous char layer with high strength on materials surface during combustion. The formed char layer effectively suppressed the heat transmission and diffusion, limited the production of combustible gases and inhibited the emission of smoke, which led to the reduction of the heat and smoke release. Moreover, the av-EHC value was obviously declined with the addition of PPBAP into EP thermosets. POLYM. ENG. SCI., 58:1858-1867, 2018. (c) 2017 Society of Plastics Engineers