Due to the poor dispersion in polymer matrix, graphene can hardly be used alone as a flame-retardant additive for polymers. In this paper, a novel halogen-free flame retardant – the ternary graft product of silsesquioxane, graphene oxide and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (PMGO) with a structure of graphene oxide layers modified with the synergistic flame retardant multiple elements of phosphorous, nitrogen and silicon was synthesized and then used in combination with traditional intumescent flame retardant (IFR) to enhance the flame retardancy of polypropylene (PP). The experimental results show that the thermal and flame retardant properties of flame-retardant (FR) PP composites are significantly improved by introducing 5 wt% PMGO as well as 20 wt% IFR. The peak heat release rate and total heat release of the FR PP composite are reduced 61.5% and 40.2%, respectively, compared to neat PP. Based on the char layer observation and thermal analysis, the enhancement in flame retardancy is mainly attributed to the outstanding intumescent char layers with high strength and thermal stability formed under the synergistic effect of PMGO and IFR. Besides, the introduced phosphorous, nitrogen and silicon hydrophilic groups do not show the negative effects on the surface hydrophobicity of flame retardant PP materials, which could broaden its scope of application.