The effect of different pre-treatment processes on the precipitation and hardening response of Al-Mg-Si-Cu was systematically studied by a combination of various characterization techniques. It was revealed that both of the pre-aging and pre-straining is unable to increase the peak aging hardness, but can accelerate the precipitation kinetics of the alloy as compared with single-stage aging. A novel multi-aging process consisted of high-temperature pre-aging (180°C for 2 h), followed by 5% pre-straining and subsequent low-temperature re-aging at 120°C can significantly increase strength without compromising elongation, achieving an ultimate tensile strength of 555.2 ± 2.7 MPa and an elongation of 11.0 ± 1.0%, ranking among the highest reported for 6xxx aluminum alloys. The combination of pre-aging and pre-straining provides sufficient nucleation site for the precipitates, while the re-aging at low temperature (120 o C) plays a key role in increasing the hardening response of the alloy due to (i) promotes gradual transformation of GP zones into β″ phase while suppressing dislocation-mediated coarsening, increasing precipitation hardening response. (ii) suppresses the dislocation recovery and thus enhances dislocation hardening. The main conclusion of this work provides new insight for increasing the hardening potential of the Al-Mg-Si-Cu alloys.