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 (SEM) were used to compare the thermal stability, flame retardancy and charring properties of EP/DOPO and EP/DOPS, and their flame retardancy mechanisms were analyzed. The results show that when the mass fraction of DOPO and DOPS is 10 %, EP/DOPO and EP/DOPS reach 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 are lower than those of pure EP, which effectively improve the flame retardancy of EP. Moreover, the char residue and flame retardancy of EP/DOPS are better than those of EP/DOPO, which is attributed to the synergistic flame retardancy 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 and TG-IR results show that DOPO and DOPS have flame retardancy in both gas phase and condensed phase, but mainly in gas phase, and DOPS has better flame retardancy in condensed phase than DOPO.