Abstract: The polyurethane flame retardant is prepared by compounding poly(diphosphophosphazene) (PDPP) and derivatives thereof, poly(diphosphate phosphazene) (MPDPP) (where M=Mg2+, Ca2+, transition metal ions, rare earth ions and the like) and poly(diphosphonic phosphazene). Since a phosphate group in the PDPP and an unreacted phosphate group in the MPDPP in the compound and an unreacted hydroxyl in the phosphate group may react with isocyanate, the flame retardant is a reactive flame retardant. Due to the reaction between the flame retardant and the isocyanate, the flame retardant is uniformly distributed in polyurethane and has a better flame-retardant effect. The flame retardant contains multiple flame-retardant components, namely polyphosphazene group, phosphate ester and phosphate salt. Due to the synergistic effect, the flame retardant has good flame-retardant properties, and can be used for various polyurethane materials.