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• The composition design of Au-Pt alloy was realized based on ultra-low MS. • Au75Pt25 is the best alloy with the lowest theoretical volume MS and A U value. • The contribution of Pt d-orbital electrons and anti-bonding states affects strength. Gold-platinum (Au-Pt) alloy has aroused considerable attention due to its ultra-low magnetic susceptibility (MS) in testing mass (TM) on spacecraft. However, the effect of Au content on the properties of the alloy has not yet been understood. In this study, the composition design of Au-Pt alloy with ultra-low MS was achieved through density functional theory (DFT) and experimental methods. The elastic, thermal properties and electronic structure were systematically investigated, the composition range was further optimized and Au75Pt25 was determined to be the most suitable alloy for TM material. The phase composition of this alloy after cold rolling and solid solution was characterized, indicating a single-phase FCC structure. In addition, there is a good validation between the experimental Vickers hardness and the DFT results. This work provides new insights into the compositional optimization of Au-Pt alloys and lays the foundation for alloy development. [Display omitted] [ABSTRACT FROM AUTHOR]