Abstract: Alkali lignin-based polyurethane foam(PUF/lignin) was prepared by substituting a part of the polyether polyol with the refined alkali lignin. The PUF/lignin/AHP material was prepared by adding aluminum hypophosphite(AHP) as flame retardant to alkali lignin-based polyurethane foam material. The flame retardant properties of PUF/lignin/AHP materials were tested by limiting oxygen index(LOI), and the thermal degradation behavior and char forming properties of the materials were investigated by thermogravimetric analysis(TG). Cone calorimetry(CONE) and scanning electron microscopy(SEM) were used to study the combustion behavior of PUF/lignin/AHP materials and the surface morphology of char residue. The test results showed that when the addition amount of alkali lignin was 5% of polyether polyol and the amount of AHP was 30%, the LOI value of PUF/5%lignin/30% AHP material reached 25.6%, and the thermal decomposition rate and total heat release of the material were lowered, and the char formation of the material was promoted. When AHP was decomposed by heat, the generated PO radicals captured the hydroxyl radicals generated during the combustion of the material, thereby suppressed the combustion reaction. At the same time, aluminum phosphate and aluminum pyrophosphate were produced to form dense carbon layer barrier substance and energy transfer, which prevented the material from further burning and improved the flame retardancy of the material.