The invention relates to a preparation method of an ammonium polyphosphate microcapsule, a polyurethane elastomer composite material and a preparation method of the composite material. The preparationmethod of the ammonium polyphosphate microcapsule comprises the following steps: adding ammonium polyphosphate and an OP-10 emulsifier into a boron-containing resin solution, and performing stirringat normal temperature until uniform emulsification is reached; dropwise adding an organic amine solution, and performing a cross-linking reaction on organic amine and boron resin; sequentially performing filtration, washing, drying and grinding after the reaction ends; and performing high-temperature heat treatment on the obtained ground product to obtain the core-shell type ammonium polyphosphatemicrocapsule. The flame-retardant polyurethane elastomer composite material prepared by using the above microcapsule flame retardant prepared through the preparation method of the ammonium polyphosphate microcapsule has the advantage of high flame resistance and excellent mechanical performances, and the microcapsule has the advantages of simple preparation process, low cost, and adaption to demands of fire safety and engineering application.