Abstract: In order to enhance the flame-retardant properties of rigid polyurethane foam (RPUF), an intumescent flame-retardant coating, formulated with chitosan (CS) and ammonium polyphosphate (APP) as precursors, was deposited on its surface via layer-by-layer (LBL) self-assembly. The effects of coating layers on surface morphology, flame retardancy, and compressive properties of RPUF were systematically investigated using scanning electron microscope (SEM), X-ray photoelectron spectrometer (XPS), laser confocal Raman spectrometer, oxygen index tester, cone calorimeter, vertical burning tester, thermogravimetric analyzer (TGA), and universal testing machine. Results indicated that the number of coating layers significantly influenced the morphological structure and flame-retardant performance of RPUF. When the self-assembled layers reached seven, the limiting oxygen index (LOI) of the modified RPUF increased from 19.5% (uncoated) to 40%, achieving UL-94 V-0 rating. Additionally, the total heat release (THR), peak heat release rate (PHRR), and total smoke production(TSP) of the flame-retardant RPUF decreased by 48.1%, 73.4%, and 36.1%, respectively, compared to the uncoated foam. The char residue was enhanced by 37.9%, while maintaining excellent compressive properties.