The effects of crystalline states, twin boundary spacing, and Fe doping on the mechanical properties of SLM aluminum are examined by performing molecular dynamics simulations of uniaxial tensile tests. The results show that the strengthening effect of twins on aluminum with different crystalline states and whether or not Fe is quite different. Inserting nano-twin in different crystalline aluminum can increase the tensile strength of single crystal aluminum, but reduce the tensile strength of equiaxed grains and columnar grains aluminum. Among them, the mechanical properties of columnar grains are the most sensitive to twins, especially the tensile strength. There is a critical value of 2.8 nm for the effect of twin boundary distance λ on the tensile strength of the columnar crystal aluminum. When λ is less than this value, the tensile strength of columnar twin aluminum decreases with the increase of λ, while when λ is greater than this value, the tensile strength of columnar grains aluminum gradually increases with the increase of λ, until it is close to that of columnar grains aluminum with twin-free. Compared with the tensile strength of different crystalline twin Al, the tensile strength of different crystalline twin Al doped with 5 at.% Fe is greater, and the strengthening effect of columnar twin Al-5%Fe is the best, indicating that reasonable control of the content of Fe and nanotwinned crystalline, which are expected to improve the mechanical properties of SLM aluminum.