Abstract: Two-pass isothermal compression tests of Al-Mg-Si-Cu alloy were conducted under deformation temperature of 653-733 K, strain rate of 0.01-0.1 s^-1 and inter-pass holding time of 30-240 s. The dynamic and static softening behavior of the alloy was quantitatively analyzed and the flow softening behavior mechanism of the alloy was investigated. The results show that the flow softening ratio of the alloy at the first deformation pass was no less than 10%; the flow softening degree decreased with increasing strain rate and deformation temperature; the softening mechanism was a combination of dynamic recovery and recrystallization. During the inter-pass periods, the flow softening behavior was obviously observed and the softening ratio reached up to 97.82%; the static softening degree increased with the deformation temperature, the strain rate and the inter-pass holding time. The softening behavior was dominated by the static recovery and recrystallization. The peak values of flow stress during the second deformation pass were relatively low and the softening ratios were no more than 5%; the softening behavior was dominated by the recrystallization and recovery.