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 solid solution （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 alloy with a copper mass fraction not exceeding 0.6% first increased and then decreased with the extension of aging time, presenting a single-peak change. The alloy with a copper mass fraction of not less than 1.2% 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 alloy was the β″ phase, and this remained unchanged after aging for 16 h. After aging at 175 ℃ for 10 h, the main precipitated phase of the high-copper alloy was the Q′ phase, and after aging for 16 h, the main precipitated phase changed to the θ′ phase. The "double peak" phenomenon of hardness of high copper content alloy was caused by the evolution of the types of precipitated phases during the aging process, and the main precipitated phase changed from Q′ phase to θ′ phase with the extension of aging time.