A new phosphorus-nitrogen flame retardant GAP – DOPO was prepared with 3-aminophenol, glutaraldehyde and DOPO as raw materials. The infrared, nuclear magnetic and TGA were employed to character the structure and thermal properties of the flame retardant. The GAP - DOPO for flame retardant epoxy resin, the TGA, SEM and Vertical combustion tester were employed to character the thermal properties, structure of carbon residue and the UL-94 of flame retardant epoxy resin; Through the above analysis, when addition 30wt % GSP - DOPO into EP, the UL-94 test was V-0; the TGA show that the carbon residue rate can be increased from 16.3% to 21.5% at 800 ℃ (N2 atmosphere), improved 31.9%; And SEM shows that the addition of flame retardant can obviously change the carbon residue morphology after combustion, increases the flame retardant properties of composite materials.