Abstract：A novel flame-retardant additive, Phenylphosphonium diaminobisethyltetraphosphonic acid (PAEPA), was synthesized from phosphoric acid, acetonitrile and phenylphosphonic dichloride(PPDC). The structure of PAEPA was characterized by FTIR, and ESI-MS. PAEPA was blended into epoxy resin to prepare flame-retardant epoxy resins. The flame-retardant and thermal properties of epoxy resins cured with triethylenetetramine (TETA) were investigated by TGA, the UL-94 test, and LOI test. The results revealed that the epoxy thermosets exhibited excellent flame retardancy and passed the V-0 rating of the UL 94 test with an LOI of 29.8% when the phosphorus content was 2.6 %. From the thermogravimetric curve, the residual carbon at 800°C is 17.7%. SEM results showed that PAEPA in the EP/TETA system obviously accelerated the formation of a stronger, phosphorus-rich, sealing char layer to improve the flame-retardant properties of the matrix during combustion.