Abstract: The thin-walled component of 5087 aluminum alloy was fabricated by cold metal transfer （CMT） arc additive manufacturing, and subsequently subjected to deep cryogenic treatment at −196 ℃ for 24 h. The influence of deep cryogenic treatment on the microstructure and mechanical properties of the thin-walled component was investigated by scanning electron microscopy, X-ray diffraction, and electron backscatter diffraction. The results show that after deep cryogenic treatment, the porosity of CMT arc additive manufactured alloy decreased, and the Al₃Mg₂ second phase was refined, and increased in quanity and distribution uniformity. After deep cryogenic treatment, the dislocation density increased. Deep cryogenic treatment decreased the aspect ratio of columnar grains, and increased the number of equiaxed grains; the overall grains were refined. After deep cryogenic treatment, the strength and impact toughness of the test alloy showed no significant changes, and the elongation after fracture increased by 31.6% and the hardness was improved by 4.7% . The plasticity enhancement of test steel was mainly attributed to the increase of dislocation density and the refinement and uniform distribution of the Al₃Mg₂ second phase.