Abstract In an effort to create an environmentally-friendly flame retardant system for rigid polyisocyanurate-polyurethane foams, expandable graphite (EG) combined with aluminum hydroxide (ATH) was firstly used to effectively improve the flame retardancy. Limited oxygen index (LOI) increased to 84.2 with an incorporation of 24 phr (parts per hundred of matrix) EG and 50 phr ATH into the matrix. Based on scanning electronic microscopy observation and thermogravimetric analysis, it was speculated that ATH could effectively induce “villi” like particles on the surface of EG, which made the intumescent char denser. The compact char layer could effectively impede the bubbles and heat transport. ATH and EG accelerated the initial degradation and fluffy char was quickly generated on the surface, which results in the slowed down degradation products of the composite and the delayed diffusion of volatile combustible fragments to flame zone.