The time-temperature-transformation (TTT) curve and time-temperature-property (TTP) curve of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy were determined by interrupted quenching technique. Phase transformations have been investigated by TEM, DSC and XRD. The result shows that under certain temperature and prolonging the holding time, the electro-conductivity of the specimens tend to increase and the hardness will decreases. Microstructure observation indicates that with the time increased, a lot of large equilibrium rod-shaped η (MgZn2)phase precipitated in the matrix and grew rapidly, which results in loss of solutes in the aging process and weakened the strengthening effect. The main reasons for precipitation of η particles are that the rapid diffusion of solute atoms and the strong driving force for phase transformation. Quench sensitive temperature range is 270℃~390℃. Consequently, the cooling rate should?be consider?lowering appropriately from the solution temperature to 390℃ to control the residual stress. And then, in the quench sensitive range, it is necessary to increase the cooling rate properly to obtain high mechanical properties.