Abstract: The constituents in the semicontinuous casting ingot of the super-high strength Al-6.5Zn-2.4Mg-2.3Cu aluminum alloy and how they change in the process of homogenization treatment at 470°C were studied by differential scanning calorimetry (DSC), optical microscopy (OM) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). The results showed that non-equilibrium freezing eutectic containing Al, Zn, Mg, Cu in the ingot not only dissolves into the alloy matrix, but also transforms into S phase (Al2CuMg) in the homogenization treatment process, and the S phase thus formed melts at 490°C in this alloy. During the initial period of the process, the formation rate of S phase is higher than the rate of S phase's dissolution into the alloy matrix, thus resulting in that the quantity of S phase increases at first then decreases to equilibrium value with the prolonged time for homogenization. However, the S phase in the alloy can not be dissolved into its matrix completely.