Abstract: The variation of dislocation density has an important influence on the mechanical properties of the material and the precipitation characteristics of the strengthening phase. The dislocation density of Al-Zn-Mg-Cu alloys after severe plastic deformation was studied theoretically and experimentally by analyzing X-ray diffraction（XRD） results and combining with the corresponding theoretical model, and the dislocation density was calculated to be about 3.87×10¹⁴/m~2. The results show that the effect of dislocation density on XRD full width at half maximum（FWHM） is mainly caused by grain refinement and lattice microstrain.