In this work, the A18B4O33w (ABOw) was in-situ generated on the surface of Al2O3 through the reaction of B2O3 and Al2O3, resulting in the thorn structure. Then, the ABOw@Al2O3/AZ91 composites were fabricated through a combination of freeze casting and pressure casting. The influence of the initial content of B2O3 on the microstructure and mechanical properties of ABOw@Al2O3/AZ91 composites was investigated. The results show that the lamellar structure of ABOw@Al2O3/AZ91 composites were weakened, which gradually changed to dendritic structure with the increasing B2O3 content. The in-situ formation of ABOw on the surface of Al2O3 was favorable to improve the compressive strength of both ABOw@Al2O3 ceramic scaffold and ABOw@Al2O3/AZ91 composites, while also reducing the anisotropy of the composites. Furthermore, the in-situ generated ABOw within the ceramic layer could effectively relieve stress concentration and delay crack initiation and propagation, which led to the enhancement of the load-bearing capacity of ABOw@Al2O3/AZ91 composites. The ABOw@Al2O3/AZ91 composite exhibited the outstanding mechanical properties with the compressive strength of ∼ 685 MPa, when the B2O3’s content is 5 wt.%.