The Al-Zn-Mg-Cu alloy with high Zn content was cast at different temperature gradients by directional solidification. The primary dendrite arm spacing λ₁, the secondary dendrite arm spacing λ₂, and the Vickers hardness of specimens were characterized. Based on the experiment results, the relationship among temperature gradient, dendritic arm spacing, and microhardness was determined by linear regression analysis and curve fitting analysis. The results are in agreement with the dendritic growth theoretical models, and the solidification parameters of Al-Zn-Mg-Cu alloy were obtained. In addition, the influence mechanism of temperature gradient on microhardness was analyzed. The results have a guidance function on the optimization of preparing methods of Al-Zn-Mg-Cu alloy with high zinc content.