6(4-of DOPO hydroxymethyl phenoxy ) cyclotriphosphazene (DOPOMPC) and ammonium polyphosphate (APP) were mixed and the mixture was further added to epoxy resin (EP) to prepare a basis expansion flame retardant composite (DOPOMPC/APP/EP). An organic modified montmorillonite (OMMT) was then added to the composite in order to further improve the flame retardant and mechanical properties of the epoxy resin. The synergistic effect of OMMT and DOPOMPC were studied by limiting oxygen index (LOI) measurement, UL-94 horizontal burning tester (UL- 94), cone calorimeter (CONE), SEM observation and other methods. Results showed that the EP4 (10%DOPOMPC/10%APP/3%OMMT/EP) was the best with comprehensive combustion parameters, such as its peak heat release rate (pk-HRR), average effective heat of combustion (av-EHC), average extinction area (av-SEA) and carbon monoxide average release rate (av-CO) could be reduced by 78.4%, 62.2%, 57.8% and 50.0% respectively of those for pure EP, besides, its LOI value reached 36.8% as well. This kind of composite showed good performance in flame retardance and smoke suppression. Furthermore, the addition of OMMT may facilitate the formation of a much compact and hard carbon scale on the surface of the EP4 during the initial stage of combustion.