Abstract: The lignin was liquefied using both polyethylene glycol and glycerol as the solvent under acidic conditions. Phospho-containing lignin(PL) was prepared by modifying the liquefied lignin with phenylphosphine dichloride. The rigid polyurethane foam of PL/ammonium polyphosphate(PLF) was synthesized using one-step foaming method by esterification reaction of polyether polyols with polyphenylmethane polyisocyanate on the basis of adding an equal amount of ammonium polyphosphate, in which part of polyether polyols was replaced with PL. The effect of PL content on the mechanical properties, thermal properties, and fire resistance of the foam materials were evaluated by comprehensive analysis, including phosphorus content, fourier transform infrared spectroscopy(FT−IR), scanning electron microscopy, compressive properties, apparent density, limiting oxygen index, thermal conductivity, and total smoke density release. The results indicated that the characteristic absorption peaks of FT−IR curve at 1350, 1037 cm⁻¹, and 1200 cm⁻¹ were corresponds to P=O stretching vibration peak, P—O—C(aliphatic) and P—O—C(aromatic), respectively, which confirmed the lignin was successfully grafted with phenylphosphine dichloride. The polyurethane foam gradually changed from white to yellowish brown, thermal conductivity decreased from 0.073 W/(m·k) to 0.055 W/(m·k), the performance of heat preservation improved significantly, and compressive strength and water absorption rate first increased and then decreased with the substitution of PL gradually increasing from 0% to 34%.