Polyurethane materials have good chemical resistance and mechanical properties, which are widely used in many fields. However, the characteristic of easy combustion limits their further application. In order to obtain polyurethane composites with both flame retardancy and good mechanical properties, ammonium polyphosphate (APP) was surface modified with arginine (Arg) to improve their flame retardancy and thermal insulation performance against hard polyurethane. The results showed that when the content of Arg-APP was 10%, the LOI value of the polyurethane composites was maintained at around 27%, passing the UL-94 V0 level. The total heat release value of the polyurethane composites was maintained at 40 MJ/m², the carbon residue rate was maintained at around 25%, and the tensile strength and bending strength were 58.23 MPa and 7.22 MPa, respectively. The elongation at break was maintained at around 7.39%. Overall, the appropriate addition of Arg-APP can effectively improve the flame retardancy and anti dripping performance of polyurethane, allowing polyurethane composites to have a longer ignition time and lower total heat release. It also effectively improves the carbon forming effect of polyurethane composites and enhances their thermal stability in high-temperature environments. Arg-APP not only improves the tensile strength and bending strength of polyurethane composites, but also effectively improves the elongation at break of the material.