Abstract: The cyclic crosslinking poly(cyclotriphosphazene-co-4,4'-thiobisphenol) (PTP) micro-nano spheres were synthesized using hexachlorocyclotriphosphazene (HCCP) and 4,4'-thiobisphenol (TDP) as raw materials. The obtained PTP spheres were applied in flame retarded epoxy resin (EP) to prepare PTP/EP composites. The PTP spheres were characterized by FTIR and SEM. The thermal stability of PTP/EP composites was investigated by TG, and the flame retardancy was carried out by limiting oxygen index test (LOI) and cone calorimetry (CONE) tests. The results show that the PTP spheres are neither soluble in any organic solvents nor meltable at high temperature. The sphere size is about 500 nm-3 μm and possesses excellent thermal stability and charring ability. The initial decomposition temperature is as high as 453.2℃ and the char residue is 74.3% at 800℃. The addition of PTP spheres can significantly improve the flame retardancy of EP. The peak heat release rate (PHRR) of PTP/EP composite is reduced by 55.43% and the LOI value is increased from 25.6% to 30.4% when merely 5wt% of PTP added. The addition of PTP microspheres also enhanced the mechanical strength of EP. The mechanical strength of PTP/EP composites is also improved with the incorporation of PTP spheres. The PTP spheres can act as a reinforcing agent at room temperature due to the insolubility and infusibility, and also acts as the flame retardant under combustion. The study provides a new ideas and methods for functional flame retardants.