A novel flame retardant additive, DOPO-based polymeric phosphate (PFR-D), which simultaneously contained phosphorus and sulfur, was synthesized from 9,10-dihyro-9-oxa-10-phosphaphnanthrene-10-oxide (DOPO), POCl3 and bisphenol S. And the structure of PFR-D has been characterized by 1H-NMR, 31P-NMR and FT-IR. PFR-D was used as additive in polycarbonate/acrylonitrile-butadiene-styrene alloy (PC/ABS). The UL94 V-0 rating was achieved by addition of 5-7% PFR-D in PC/ABS, the LOI reached 27.5%. The thermal degradation of PFR-D and polymers with it was investigated by TGA, and the results showed that addition of PFR-D apparently changed the pyrolysis pathways of PC/ABS. The TGA curves indicated that the flame retardant effect was attributed to promoting the char yield by involving the polymer in charring.