The novel intumescent flame retardant rigid polyurethane foams ((PEPS‐RPUF) were prepared based on a P‐N containing intumescent flame retardant, poly ethanediamine spirocyclic pentaerythritol bisphosphonate (PEPS). The flammability, thermal, and mechanical properties of PEPS‐RPUF were discussed. Scanning electron microscopy (SEM) and compression strength tests showed PEPS exhibited favorable compatibility with polyurethane matrix and smaller negative influence on the mechanical properties of PEPS‐RPUF. Cone calorimeter, vertical burning test (UL‐94) and limiting oxygen index (LOI) showed when the content of PEPS was 25 pph, the peak heat release rate (PHRR) and total heat release (THR) decrease, respectively, from 75.2 to 52.2 kW m −2 and from 5.7 to 4.1 MJ m −2 . The LOI of PEPS‐RPUF could reach 27%, and a UL‐94 V‐0 rating was achieved. The PEPS‐RPUF exhibited an outstanding water resistance that it could still obtain a V‐0 rating after water soaking. Thermogravimetric analysis (TGA) showed the charring ability of PEPS‐RPUF was improved greatly compared to RPUF. SEM and Fourier transform‐infrared spectroscopy (FT‐IR) were utilized to characterize the surface morphology and chemical structure of the intumescent chars formed from PEPS‐RPUF. The results indicated the chars were compact and smooth, which was a critical factor for protecting the substrate material from burning. POLYM. ENG. SCI., 53:2478–2485, 2013. © 2013 Society of Plastics Engineers