A prediction model for microstructure evolution and ageing strengthening was developed for Al-Cu-Mg-Ag alloys with high Cu/Mg ratio before peak ageing stage. Firstly, the expression of critical parameters was built for the quantitative relationship of Mg and Mg-Ag atom clusters to consumed Cu atoms during Ω phase precipitation, i.e. the interface energy model of Ω precipitates and the substrate. Provided the alloy is pseudo-binary approximation, quantitative distribution model of Cu solute atom in the two strengthened phases for the Al-Cu-Mg-Ag alloy was set up based on the thermodynamic model and the classical nucleation and growth theories. The model can predict the evolution of the radius and precipitation density of two strengthened particles and the solute concentration with temperature and time. Finally, the strengthening model based on Orowan equation was solved. The microstructure evolution and strength prediction of the model were generally in good agreement with the experimental data, which verified the reliability of the models.