The effects of Mg element on the distribution of dislocations in Al-Cu-Mg alloy have been investigated. It has been shown that the interaction between Mg and Cu atoms was decreased since the concentration of Cu in Al matrix decreases due to the formation of Al2Cu precipitates during deformation and aging. The Mg element blocks the distributed glide of dislocations and thus inhibits the formation of cell structures due to the large atom radius, producing high aberrance energy and internal friction in Al matrix. Compared with the cell structures in typical metals and alloys with high stacking-fault energy, the dislocations in Al-Cu-Mg alloy are quasi-uniformly distributed and can decrease the free energy, named as the Taylor lattice distribution.