Abstract: A new intumescent flame retardant (IFR) system, which was composed of a flame retardanted oligomer (PSPTR) and phenol formaldehyde resin (PF), was used to impart flame retardancy for ABS resin. The flammability properties of PSPTR-PF/ABS composites were investigated by limiting oxygen index (LOI) and vertical burning test (UL-94), respectively. The results show that the LOI value of PSPTR-PF/ABS composites reach 28.2, and UL-94 reached V-1 rating when the total addition mass fraction is 30%, in which the mass ratio of PSPTR to PF is 1:1. The thermal property and degradation processes of ABS and PSPTR-PF/ABS composites were studied by thermogravimetric analysis-infrared spectrometry (TG-IR) in detail. In addition, the morphology and structure of char layer were characterized by SEM, XRD and Raman spectra. The results demonstrate that PF can improve the compactness and graphite structure of intumescent char layer, which can increase the flame retardant performance of ABS eventually.