The introduction of deformation before aging can change the aging precipitation process of the precipitation-strengthened Al-Cu-Mg(-Si) alloy, and this change is directly related to the amount of deformation. In order to study the deformation strengthening mechanism of multi-element alloys, it is necessary to use electron backscatter diffraction (EBSD), X-ray diffraction (XRD), transmission electron microscopy (TEM) and other methods to test the Al-3.0Cu-1.8 Mg-0.5Si (wt.%) alloy treated by different deformation and aging process, quantitatively or semi-quantitatively, so that the contribution of different strengthening methods to the strength is listed, which provides theoretical support for the application of thermo–mechanical treatment in aluminum alloys. It has been found that the increase in deformation can increase the peak hardness and strength of the alloy due to three reasons. One is the increase in the grain-boundary area, the second is the increase in the dislocation density, and the third is that the size of the precipitates is smaller and the distribution is more uniform. Furthermore, the contribution of precipitates to the strength is getting higher with a larger deformation prior to aging process, when the amount of deformation is greater than 6%.