The cyclotriphosphazene-based epoxy resin named as PNEP was successfully synthesized from hexachlorocyclotriphosphazene, p-hydroxybenzaldehyde and epichlorohydrin. It chemical structure was characterized by NMR spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The prepared resin materials DDM-PNEP and DDS-PNEP were cured with diamino diphenyl methane (DDM) and diamino dipheny sulfone (DDS), respectively. The flame retardant properties, thermal stability and mechanical behavior cured PNEP are investigated and the scanning electron microscopy (SEM) is used to observe the residual carbon. The PNEP cured by DDM shows better result and the LOI of DDM-PNEP is 27.1 %. The thermogravimetric Analysis (TGA) demonstrated that the initial decomposition temperature was lower than E-44 epoxy resin. The initial decomposition temperatures of DDM-PNEP and DDS-PNEP were 152 ℃ and 159 ℃ respectively. The cured PNEP had two decomposition stages, DDM-PNEP are 152 ℃～320 ℃ and 370 ℃～505 ℃, DDS-PNEP are 159 ℃～355 ℃ and 358 ℃～552 ℃. The PNEP cured with DDM had a better carbonization result, which the carbon residue rate was 54.71 % at 600℃. Morphology of the solid residues by SEM exhibit that the carbon residue was dense and uniform, this phenomenon is very helpful in the flame retardancy of the epoxy resin.