This research investigates the effect of internal defects on the tensile strength of Selective Laser Melting (SLM) additively-manufactured aluminum alloy (AlSi10Mg) test parts used for civil aircraft light weight design. A Finite Element Analysis (FEA) model containing internal defects was established by combining test data and the stress concentration factor comparison method. The effect of variation in the number, location and shape of defects on the finite element results was analyzed. Its results show that it is reasonable to use spherical defect modeling. The finite element modeling and analysis methods are also applied to the study of the effect of internal defects on tensile strength in additive manufacturing of other metallic materials. According to the FEA results of single defects at different scales, the formula for calculating the weakening degree of tensile strength applicable to the defective area of less than 15% was established. The result of the procedure is reliable and conservative. This research results can guide the selection of process parameters for the additive manufacturing of aluminum alloys. Further, the research results can promote the application of metal additive manufacturing in designing light-weight civil aircraft structures.