Over the past several decades, it was generally believed that the strength of the industrially widely used cast Al-Si-Mg-Cu alloys enhanced monotonously with increasing Cu content. However, in this study using cast Al9Si0.5MgxCu (x = 0,0.2,0.4,0.6,0.85,1.0,1.25, in wt.%) alloys under T6 heat-treated condition, it was found that the hardness and yield strength of the heat-treated alloys showed a platform in the composition range from 0.4 wt.% to 0.85 wt.% Cu, while still increased with increasing Cu content before and after the platform. With increasing Cu content, the β-Mg2Si intermetallic phase decreased and disappeared at 0.85 wt.% Cu, while the Q-Al5Cu2Mg8Si6 and θ-Al2Cu intermetallic phases increased in the as-cast alloys. After heat treatment, the micron-scale intermetallic phases were dissolved into the Al matrix and precipitated as the nanoscale β″, Q' and θ' strengthening phases. With increasing Cu content, the β″ precipitate decreased and vanished at 0.85 wt.% Cu, while the Q' and θ' precipitates increased in the heat-treated alloys. The trade-off of the phases induces the platform in the strength of the heat-treated alloys, and further increase of the Cu content in this undetected trapped platform range is not favorited industrially as it only decreases ductility.