The phosphorus-nitrogen flame retardant 6,6′- (piperazine-1,4-diyl) bis (6H-dibenzo ［c, e］［1,2］ oxophosphate-6-oxide)PiP-DOPO was synthesized using piperazine, 9,10-2H-9-oxa-10-phospaphenanthrene-10-oxide (DOPO) and carbon tetrachloride as raw materials. Its structure was characterized by FT-IR and ¹H-NMR, and PiP-DOPO was used to flame retarded epoxy resin (EP). The thermal properties of PiP-DOPO and cured products were investigated by TGA, flame retardant properties were characterized by limit oxygen index (LOI), vertical burning test (UL 94) and cone calorimeter test (CCT), and the residual carbon morphology was investigated by SEM. The results show that when the phosphorus content is 1.50% in the EP system, the carbon residue rate could reach 25.23% from 23.02% (pure EP) at 600 ℃ (in N₂ atmosphere), the LOI reaches 27.5, the UL 94 passes V-0 level, the PHRR is 325.0 kW/m², THR is 76.8 MJ/m². And the SEM result reveals that the addition of PiP-DOPO can obviously improve the carbon residue morphology after combustion. So, the PiP-DOPO can enhance the flame retardant properties of epoxy resin.