Abstract: Using the finite element analysis method, this study introduced porosity defects into SiC/AZ91D magnesium matrix composites with realistic SiC particle morphology, and analyzed the influence of different porosity rates and shapes on the mechanical behavior of SiC/AZ91D composites during uniaxial tensile process. The results show that when the aspect ratio of the pore length to width is 1, the tensile strengths of the composites with void contents of 0%, 0.5%, 1.0%, and 1.5% are 351.214 MPa, 339.452 MPa, 325.735 MPa and 306.791 MPa, respectively. The tensile strength gradually decreases with the increase of porosity rate, and the initiation and propagation time of cracks in the composite material advances with the increase of porosity rate. As the aspect ratio of the pore length to width increases, the stress concentration at the tip of the pore becomes more severe, resulting in lower tensile strength of the composite material. The crack initiation and propagation mechanism in the SiC/AZ91D composite material without porosity defects involves the initiation of microcracks at the particle-matrix interface, followed by their interconnection to form a main crack, which propagates around the particles leading to material fracture. In the case of SiC/AZ91D composites with porosity, microcracks initiate around the pores and interconnect with microcracks generates at the particle-matrix interface, ultimately converging into a main crack that propagates around the particles, causing material fracture.