The invention relates to a preform for an in-situ synthesized Al2O3 particle reinforced steel-based surface composite, a preparation method and application. The preparation method of the preform includes the steps that 1), chromium oxide powder, aluminum powder, high carbon ferro-chrome powder and borax are taken and mixed to be uniform, and mixed powder is obtained; 2), a binder is added into themixed powder, the materials are mixed to be uniform, and the mixture is pressed into a briquet; and 3), the briquet is dried in the protective atmosphere, and the preform is obtained. The preform isprepared from the raw materials of the chromium oxide powder, the aluminum powder, the high carbon ferro-chrome powder and the borax, during casting infiltration, Al2O3 particles are synthesized on the surface of a steel substrate in situ through the thermit reaction of Cr2O3 and Al, diffusion between a casting infiltration layer and the substrate is promoted through heat released through the thermit reaction, the casting infiltration layer achieves metallurgical bonding, finally the casting infiltration layer well bound with the substrate is formed on the surface of the metal substrate, richchromium carbide and Al2O3 particles are contained in the casting infiltration layer, and therefore the hardness and abrasion resistance of the casting infiltration layer can be remarkably improved.