Aluminum-based composite material is one of the most important candidate materials in the mechanical industry and aerospace engineering due to its light weight and high strength. Graphene is an ideal reinforcement for composite materials for its excellent mechanical properties. Till-now, the contribution of graphene sheets in the process of crack propagation in composites is not clear. In present work, the effects of graphene size and distribution in graphene/aluminum composites are explored using molecular dynamics simulation methods. It is found that when the length of graphene flake is less than 3.35 nm, the generated sub-cracks in the composite is benefit to the crack propagation. This effect reduces the mechanical properties of composite. When the length of graphene flake is greater than 3.35 nm, graphene sheet impedes the crack propagation and dislocates slip at sub-cracks. In addition, the distribution of graphene flakes angle changes the crack propagation path. Our findings also provide insights into ways to optimize mechanical properties of graphene/aluminum composites.