In order to investigate the effect of sulfur element on flame-retardant epoxy resin based on phosphaphenanthrene, flame retardants 9,10-dihydro-9-oxa-10- phosphaphenanthrene -10-oxide (DOPO) and 9,10-dihydro-9- oxa-10- phosphaphenanthrene -10-sulfide (DOPS) were added to EP according to the mass fraction of 5%, 7.5% and 10% respectively, and 4,4 '- diaminodiphenylsulfone (DDS) was used as curing agent to prepare flame retardant composites. Thermogravimetric analysis (TG), limiting oxygen index (LOI), vertical flame test (UL-94), cone calorimeter test (CONE), thermogravimetric-infrared spectroscopy analysis (TG-IR) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) were used to compare the thermal stability, flame retardancy and charring properties of two Flame Retardant Composites EP/DOPO and EP/DOPS, and their flame retardancy mechanisms were analyzed. The results showed that when the addition of DOPO and DOPS was 10%, EP/DOPO and EP/DOPS reached UL-94 V-0, with LOI values of 31.0% and 30.6% respectively; The heat release rate (HRR), total heat release rate (THR) and average mass loss rate (av-MLR) of the two composites were lower than those of pure EP, which effectively improved the flame retardancy of EP. Moreover, the carbon residue and flame retardancy of EP/DOPS were better than that of EP/DOPO, which was attributed to the synergistic flame retardance of P and S elements in DOPS, which could better promote the dehydration of substrate into carbon and enhance the flame retardancy of condensed phase. SEM-EDS and TG-IR results showed that EP/DOPS had better charring ability and carbon layer quality than EP/DOPO. DOPO and DOPS had flame retardancy in both gas phase and condensed phase, but mainly in gas phase, and DOPS had better flame retardancy in condensed phase than DOPO.