Al-Cu-Mg aluminum alloy sheet is widely used for aircraft skin. Multi-stage forming combined with Solution Heat Treatment (SHT) is necessary for some geometrically complex productions in aircraft skin components. However, the effects of pre-strain and natural ageing time on the formability of Al-Cu-Mg aluminum alloy sheet are not fully understood. In this study, the deformation responses of an Al-Cu-Mg aluminum alloy sheet, over a range of pre-strain levels (0% to 6%) and natural ageing time levels (0 h to 2 h) were investigated. A modified Hollomon constitutive model coupling with pre-strain and natural ageing time was proposed to characterize the deformation responses of the Al-Cu-Mg aluminum alloy sheet. The hemispherical dome tests were performed at natural ageing time levels ranging from 0.25 h to 1.5 h. Marciniak and Kuczynski (M-K) theory was used to obtain the forming limit prediction model which takes pre-strain and natural ageing time into account. The theoretical Forming Limit Curves (FLCs) were compared with the experimental results.