<p>Al-Si-Cu-Mg alloy demonstrates a significant age-hardening effect, with its mechanical properties tunable by optimizing the aging parameters. To enhance this effect, the as-cast Al-8.5Si-2Cu-0.9Mg alloy was subjected to either single-stage aging at temperatures of 150 °C, 175 °C, 200 °C, and 225 °C for 0.5 h to 20 h; or double-stage aging: involving a first-stage aging treatment at 120 °C for 1 h, 3 h, 5 h, or 7 h, followed by a second-stage aging treatment at 175 °C for 0.5 h to 20 h. The microstructure and mechanical properties were evaluated for samples aged at 175 °C/7 h, 175 °C/10 h, 120 °C/5 h+175 °C/5 h, and 120 °C/5 h+175 °C/8 h. XRD analysis reveals that the as-cast Al-8.5Si-2Cu-0.9Mg alloy consists of the α-Al, Si, θ-Al₂Cu, and Q-Al₅Cu₂Mg₈Si₆ phases. The aging kinetics exhibit a double-peak behavior in both single-stage and double-stage aging processes. Under single-stage aging at 175 °C/<i>x</i> h and double-stage aging (120 °C/5 h+175 °C/<i>x</i> h), the precipitates’ size at the first peak is smaller than that at the second peak. Compared with single-stage aging (175 °C/7 h), double-stage aging (120 °C/5 h+175 °C/5 h) produces a finer precipitate in the alloy. Theoretical calculations indicate that the number density and nucleation rate of both the Al₅Cu₂Mg₈Si₆ and Al₂Cu phases are higher during the double-stage aging (120 °C/5 h+175 °C/5 h) than those during the single-stage aging (175 °C/7 h). Additionally, tensile tests at both room temperature and 250 °C demonstrate that double-stage aging (120 °C/5 h+175 °C/5 h) significantly improves the mechanical properties of the alloy compared to single-stage aging (175 °C/7 h), suggesting that double-stage aging is more effective for enhancing mechanical properties for this alloy.</p>