Epoxy resins (EP) are widely used in chemical corrosion protection, electronic devices, and construction materials due to their good comprehensive properties. However, the inherent flammability and brittleness limit its application in the fields requiring high- performance. In this paper, a DOPO- containing sulfathiazole compound (FSTD) was synthesized using furfural, sulfathiazole and 9, 10-dihydro-9-oxa-10-phospha- phenanthrene-10-oxide (DOPO) as raw materials, and the EP/FSTD composites were prepared by solution method. The effects of FSTD on the thermal stability, curing behavior, flame retardancy, combustion behavior, mechanical properties and transparency of EP/FSTD composites were systematically studied. The composite containing 4% FSTD achieves a UL-94 V-0 rating with a limiting oxygen index value of 35.2%, and the peak heat release rate, total heat release and total smoke release are reduced by 35.9%, 37.3% and 71.5%, respectively. The addition of FSTD not only promotes the formation of continuous dense carbon layers, but also releases non-flammable gases and phosphorus-containing free radicals, which have the flame retardant effect of condensed phase and gas phase. Compared to pure EP, EP/4% FSTD composite maintains the same level of transparency while the impact strength is almost doubled.