Low-activity amine chain extenders and low-activity isocyanate componentswere introduced to regulate the reaction rate and mechanical properties of polyurea coatings（PUA）. Combined with a novel starch-based phosphate melamine salt flame retardant（SPM）a deactivated flame retardant polyurea coating was prepared on the surface of the polyurethane-，based expansion materials（PUF）. The flame retardancy and mechanical properties of the modified-PUF materials were investigated through characterization methods such as extreme oxygenindex，vertical combustion method，and compressive and flexural tests. The research resultsindicated that the modified PU/PUA10S successfully passed the UL-94 test，reached the V-0，and had a good flame retardant effect. Compared with the initial PUF，the mechanical properties of PU/PUA10S were significantly improved，with compressive strength increasing from 3. 4 MPato 5. 0 MPa and flexural strength increasing from 2. 5 MPa to 3. 4 MPa. In this study，a surface-enhanced flame retardant protective material in situ was prepared，which could be applied in fields such as emergency repair/construction and emergency protection.