Carbon nanosphere based nitrogen-phosphorus-sulfur composite flame retardant (CNSs-H-D) was synthesized with the desired carbon nanospheres (CNSs) as core, hexachlorocyclotriphosphazene (HCCP) as bridge and amino diphenyl sulfone (DDS) as grafting agent. The morphology and thermal stability of CNSs-H-D were characterized. Afterwards, the composite of CNSs-H-D reinforced epoxy resin (EP) (CNSs-H-D/EP) was made and its flame retardancy and the relevant mechanism were investigated. Results show that the synthesized CNSs-H-D displayed a spherical morphology with a mean diameter of 80 nm and excellent thermal stability. Specifically, the LOI of CNSs-H-D/EP increased from 20.0% (of pure EP) to 27.5% (with addition of 5%(mass fraction) CNSs-H-D/EP), thus its flame retardance reached grade V-2 and its peak value of heat-release rate and fire-risk index reduced by 16.8% and 42.2%, respectively. Moreover, CNSs-H-D could significantly improve the thermal stability and char formation of EP. The initial decomposition temperature and high-temperature char residue of CNSs-H-D/EP increased by 40℃ and 144.7% respectively, in comparison with the pure EP. Furthermore, the initial weight loss temperature of CNSs-H-D/EP was 190℃ higher than that of EP. With good compactness and continuity of char layer, the CNSs-H-D/EP exhibited a typical condensed phase flame-retardant mechanism, where the char residue was as high as 94.5% at 800℃.