Three Al-xMg-yZn-1Cu alloys with equal total alloying element contents but different Mg/Zn ratios have been prepared in this work, i.e., Al–6Mg–1Zn–1Cu, Al–5Mg–2Zn–1Cu and Al–4Mg–3Zn–1Cu alloys. The influences of Mg/Zn ratios on microstructure and mechanical properties of heat-treated Al-xMg-yZn-1Cu alloys have been systematically studied. The results show that Al-xMg-yZn-1Cu alloys with higher Zn contents exhibit the faster aging response as Zn atom features the largest diffusion coefficient. Additionally, rod shaped and ellipsoidal T′ precipitates are observed in the Al–6Mg–1Zn–1Cu alloy. While in Al–4Mg–3Zn–1Cu alloy under TMT-2 process, T′ precipitates mainly exist in an ellipsoidal shape, with a higher number density (∼1.79 × 1023 m-3) and smaller size (∼4.1 nm) compared to the Al–6Mg–1Zn–1Cu alloy. The optimized thermomechanical treatment (80 °C/5 h + rolling + 130 °C/x h) could effectively improve the yield strength of Al–5Mg–2Zn–1Cu and Al–4Mg–3Zn–1Cu alloys, with values increasing from ∼419 MPa to ∼441 MPa–∼454 MPa and ∼470 MPa, respectively. In contrast, the Al–6Mg–1Zn–1Cu alloy exhibit almost no increment in yield strength after the same thermomechanical treatment. This could mainly be attributed to the higher density of T′ precipitates in alloys with higher Zn contents, resulting from the higher nucleation rate of T′ precipitates caused by high contents of Zn atoms featuring fast diffusion rates, with the assistance of dislocations introduced by rolling. This work provides theoretical supports for the development and production of age-hardenable Al-xMg-yZn-1Cu alloys.