The microstructure and interface diffusion behaviour of SiC_P/Al composites prepared by high-pressure torsion under different turning numbers were investigated by OM and EDS, and the effect of turning numbers on the tensile properties and fracture mechanism was analysed combined with microstructure and interface characteristics. The results show that increasing the turns can improves the uniformity of the particles and reduces the porosity, with the Al diffusion ability enhanced and the diffusion distance increased, and the diffusion coefficient of Al is enhanced by about 10¹⁷ times in comparison with the theoretical value, leading to tensile strength and elongation of composites improved continuously and the fracture is mainly based on the ductile fracture of matrix, with a small amount of SiC particles distributed uniformly in the dimple of the fracture. When the turns come to bigger, SiC particles are broken severely by the shearing deformation, particles "regeneration and agglomeration" leads to increased porosity and more potential crack sources, forming a large quantity of new fracture interfaces with low bond strength. The tensile strength and elongation of the sample decrease greatly, and it is easy to form larger deep dimples in the agglomeration, composite fracture exhibits the mixed mode of ductile fracture and brittle fracture.