Based on the real microstructure of composite materials and introducing cohesive element units at the interface between particles and matrix, four finite element models with different particle aggregation distributions (uniform distribution, aggregation at three locations, aggregation at two locations, and aggregation at one location) were established to investigate the influence of particle aggregation on the crack initiation and propagation mechanisms of SiC/AZ91D composite materials. The results show that when the crack initiates, stress distribution in the matrix is highly uneven, with the maximum stress occurring at the corners of the particle group. The more severe the particle aggregation, the greater the maximum stress value during crack initiation. As the crack propagates, the greater the degree of particle aggregation, the higher the maximum stress value in the matrix and the greater the extent of crack propagation. When the crack completely fractures, the maximum stress value of the particles gradually increases with the aggravation of particle aggregation, while the maximum stress value of the matrix remains relatively constant. Particle aggregation accelerates the process of crack initiation and propagation, and particles are uniformly distributed in the matrix. The crack initiation and propagation mechanism of composite materials is due to the severe stress concentration at the boundaries and corners of the SiC particle group, which causes damage to the matrix, initiates microcracks, and then propagates along the direction of maximum shear stress to form the main crack.