Three kinds of Al-5Sr-5Sb modifiers were prepared by permanent mold casting (PMC), controlled diffusion solidification (CDS), and rolling. The influence of the preparation method on Al-5Sr-5Sb was examined by assessing the melting enthalpy from DSC curves and conducting first-principles calculations. Al-5Sr-5Sb was incorporated into AlSi10MnMg alloy to examine the modification effect and mechanism. It is found that the AlSi10MnMg alloy modified with CDS-prepared Al-5Sr-5Sb shows significant improvement in its microstructure, changing the eutectic Si phase from lamellar to coral-like or fiber-like structures. This modification leads to enhanced macroscopic properties: the thermal conductivity is increased by 24.14% to 189.63 W/(m·K), elongation is increased by 59.15% to 10.87%, and tensile strength is increased by 66.9% to 254.98 MPa compared to the unmodified alloy.