<p>Rigid polyurethane foam (RPUF) was prepared with walnut shell powder as the carbon source and ammonium polyphosphate (APP) as the acid source. Its thermal stability, flame retardancy, and mechanical properties were investigated by thermogravimetric analysis, cone calorimetry, scanning electron microscopy (SEM) analysis, limiting oxygen index (LOI) test, and compression test. The result showed that the foam with 5&#xa0;wt.% of walnut shell and 10&#xa0;wt.% APP (RPUF-W5APP10.0) had the highest thermal decomposition temperature and activation energy (E). It showed that the addition of APP improved the thermal stability of the RPUF. RPUF-W5APP10.0 had the lowest peak heat release rate (PHRR) and total heat release (THR), which were 57.98 and 38.74% lower than the foam without APP (RPUF-W5APP0) at 25&#xa0;kW/m². It also exhibited the highest LOI (24.4%) and UL-94 rating (V-0). In addition, RPUF-W5APP10.0 had the lowest peak smoke production rate (PSPR) of 0.0050&#xa0;m²/s and total smoke release (TSR) of 43.29&#xa0;m²/m². SEM and Raman analyses revealed that RPUF-W5APP10.0 had the densest and most structurally intact char layer. The mechanical properties and thermal insulation of RPUF-W5APP10.0 were far superior to RPUF-W5APP0. The study provides a useful reference for the expansion of the flame-retardant field of bio-based RPUF.</p> Graphical abstract <p></p>