Pore defects adversely affect the mechanical properties of laser-additive manufacturing alloys. To elucidate the effect of pore defects on the tensile properties of Al-Mg-Sc-Zr alloy formed by selective laser melting (SLM), this study characterizes the internal pore defects of the alloy in three dimensions using X-ray computed tomography and obtains the tensile properties of the alloy through room-temperature tensile tests. Based on the pore data and tensile-test results of the Al-Mg-Sc-Zr alloy, a representative volume element (RVE) model that reflects the constitutive and pore characteristics of the material is established. Assuming an unchanging matrix of the Al-Mg-Sc-Zr alloy formed by SLM, the tensile properties of the alloy are evaluated for different porosities and pore sizes in the RVE model. The calculations show that the tensile strength and elastic modulus of the alloy decrease obviously when the porosity increases. And, when the pore size increases, the tensile strength of the alloy decreases significantly, but the elastic modulus does not change significantly. When the pore size exceeds 100 μm, obvious stress concentration appears around the pores.