In order to expand the application scenarios of polyurethane foam, a one-step coating process was used in this study to prepare biomass-based polyurethane foam insulation material with co-efficent flame retardancy by coating a thin layer of lignin flame retardant on polyurethane foam substrate and adding expanded graphite. The results show that the physical and mechanical properties, thermal stability and flame retardancy of polyurethane foamed plastics are greatly improved by adding expanded graphite and coated with lignin flame retardant, and the thermal conductivity is slightly increased. When expanded graphite was added to the substrate and lignin flame retardant with a mass concentration of 30% was coated, the density of biomas-based flame retardant polyurethane foam could reach 45.3 kg/m³, its compression strength was increased by 233%, the thermal conductivity of the material was increased by 55.6%, the initial thermal decomposition temperature was 291 ℃, and the mass retention was increased by 248%. The material oxygen index was increased to 33.7%, the peak heat release rate was 83.2 kW/m², the time to reach the peak heat release rate was 21.2 s, and the material fire safety factor was greatly increased. The results show that the addition of expanded graphite and thin coating with lignin flame retardant can significantly improve the flame retardancy of the material.