Abstract: Al-Si-Mg-Cu alloys with different copper mass fractions （0, 0.6%, 1.2%, 2.1%） were cast and subjected to T6 heat treatment of two-stage solution treatment （500 ℃×6 h+530 ℃×4 h） + water quenching + aging at 175 ℃ for 0-96 h + air cooling. The influence of copper content on the aging precipitation behavior of the alloy was studied. The formation mechanism of the "double peak" phenomenon in hardness was analyzed. The results show that the hardness of the low copper content （not exceeding 0.6%） alloy first increased and then decreased with the extension of aging time, presenting a single-peak change. The high copper content （exceeding 0.6%） alloy exhibited a "double peak" phenomenon in hardness, and the second hardness peak was slightly higher than the first one. After aging at 175 ℃ for 10 h, the main precipitated phase of the low copper content alloy was the β″ phase, while that of the high copper content alloy was the Q′ phase. After aging at 175 ℃ for 16 h, the main precipitated phase of the low copper content alloy remained unchanged, while that of the high copper content alloy was the θ′ phase. The "double peak" phenomenon of hardness in high copper content alloys was caused by the evolution of the types of precipitated phases during the aging process, and the main precipitated phases changed from Q′ phase to θ′ phase as the aging time increased.