The shear, transverse mechanical properties, and durability of composite materials largely depend on the interfacial strength between the fibers and the matrix. The interfacial strength of composites typically relies on the size, shape, properties, and spatial distribution of the fibers and resin matrix. The paper proposes a numerical simulation method suitable for composite materials. Short-chopped glass fiber composites were chosen as the research object for finite element simulation. A fast generation algorithm for the random distribution of fibers is provided, and the influence of short-chopped glass fiber length on the mechanical properties of sheet molding compounds is analyzed. The results show that the elastic modulus of short-chopped glass fiber sheet molding compounds increases first and then levels off with the increase in fiber length. By comparing random and regular structures, the rationality of the calculation method is demonstrated. The randomness of fiber distribution has a relatively small impact on the macroscopic elastic constants of composite materials.