Abstract: Epoxy resin (EP) was flame retarded with halogen-free intumescent flame retardant six-(4-DOPO hydroxymethyl phenoxy) cyclotriphosphazene (DOPOMPC), polyphosphate (APP) and multi-walled carbon nanotubes (MWCNTs) to prepare new flame retardant composites DOPOMPC-APP-MWCNTs/EP. The flame retardant property of the composites was analyzed by limit oxygen index (LOI), horizontal vertical combustion, and cone calorimetry. The result demonstrates that the addition of MWCNTs increases the flame retardant property and mechanical property of intumescent flame retardant system, and improves the dense smoke phenomenon to a certain extent when system is burning. When the total mass fraction of flame retardant system is 20% and the mass fraction of MWCNTs is 2%, optimal properties of materials can be obtained. The LOI of the composites is 36.8%, the peak heat release rate, average effective heat of combustion, average specific extinction area, average CO release rate are respectively reduced by 83.5%, 31.5%, 47.6%, 50.0% compared with the non-flame retardant EP, and reduced by 83.5%, 77.7%, 83.7%, 68.9% compared with DOPOMPC-APP/EP. SEM analysis reveal that a dense, cross-linked network charred layer is formed in DOPOMPC-APP-MWCNTs/EP after the addition of MWCNTs.