The yield strength and work hardening properties of an Al-Cu-Li alloy AA2050 after artificial ageing have been experimentally investigated and modelled in this study. Uniaxial tensile stress-strain curves of the alloy artificially aged for up to 500 h have been acquired and evolutions of main precipitates during ageing have been summarised to elucidate the underlying mechanisms of the observed mechanical properties, such as yield strength and work hardening behaviour. Work hardening analysis with Kocks-Mecking plots has been performed to analyse the shearing-to-bypassing transition progress of the aged alloy and it has been found that the transition does not occur at the peak-ageing state. A new mechanism-based unified constitutive model, comprising three sub-models, has been developed to simultaneously predict the evolutions of microstructures, yield strength and work hardening properties of the artificially aged AA2050. It is the first unified model covering a wide range of artificial ageing conditions from under-ageing to over-ageing, providing an effective tool for performance prediction of the aged alloys for industrial applications. The model has the generic feature and could be applied to artificial ageing of other 2xxx series aluminium alloys with dominant T1 precipitates.