The effects of the Cu content on the microstructure and strengthening mechanisms of the Al-Mg-Si-xCu alloys were systematically investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and mechanical tensile tests. The results show that, the strengthening mechanisms change with the Cu content. For as-quenched alloys, solution strengthening (σ_SS) is predominant when the Cu content ≥2.5 wt.%, and of equivalent importance as grain size strengthening (σ_H-P) when the Cu content ≤1.0 wt.%. With respect to peak-aged alloys, precipitation strengthening (σ_ppt) is predominant when the Cu content ≥2.5 wt.%, but σ_SS becomes predominant when the Cu content is 4.5 wt.%. As the Cu content increases from 0.5 to 4.5 wt.%, the main type of precipitates in alloy tends to change from a β″ phase to Q′ phase, and then to a θ′ phase. Among the three types of precipitates, θ′-precipitate causes the largest increase in yield strength (σ_0.2) and the largest decrease rate in elongation. β″-precipitate leads to the smallest increase in σ_0.2 and the smallest decrease rate in elongation. The increase of Cu content reduces Si solubility in the Al matrix and thus decreases the nucleation rate of β″ phase during subsequent aging.