Acrylonitrile-butadiene-styrene (ABS) was treated with various formulations containing an intumescent fire retardant, which consists of ammonium polyphosphate (APP) and a novel char-forming agent, poly(diphenolic phenyl phosphate)(poly(DPA-PDCP)). The behaviour of this intumescent system was investigated by thermogravimetric analysis (TGA), LOI test and cone calorimeter test, respectively. The results showed that the addition of poly(DPA-PDCP) enhanced the thermal stability and flame retardancy of ABS/APP. The weight of residues improved with the addition of poly(DPA-PDCP) . SEM investigations of residual char burning after cone calorimeter test revealed that poly(DPA-PDCP) plays an stimulative role in the process of carbonization. The intumescent chars formed from ABS/APP/ poly(DPA-PDCP) composites were intact and strong. It is confirmed that the poly(DPA-PDCP) is an efficient char-forming agent for flame retardant ABS resin.