In the current study, the evolution of S(Al₂CuMg) phase in a commercial Al-Zn-Mg-Cu alloy during homogenization, deformation and solution treatments was investigated and the effect of residual S(Al₂CuMg) phase on mechanical properties in finished plates was revealed. The results showed that S(Al₂CuMg) phase formed during solidifications and existed in eutectic structure. Parts of AlZnMgCu phase transformed to S(Al₂CuMg) phase after the first homogenization treatment with a regime of 470°C/24h. During the second homogenization treatment at 480°C, most of the S phase had gradually dissolved. However, a handful of large-sized S phase remained even after extending the second stage homogenization time to 48h. During deformation these remaining S(Al₂CuMg) phase was broken, resulting in size reduction and apparent distribution along the rolling direction, which could be effectively eliminated by a two-stage solid solution regime of 470°C/1.5h+480°C/2h. Comparing the microstructure and properties of final plates treated by homogenization processes, it was evident that the alloy subjected to a single-stage homogenization treatment exhibited a significantly higher amount of residual S phase, which directly compromised fracture toughness but has a relatively minor impact on strength.