Abstract: This study used the layer-by-layer UIT process to reinforce the Al-5%Mg alloy thin-walled parts through CMT-WAAM. The effects of different ultrasonic impact currents on macroscopic forming, pore evolution, solidification microstructure, and mechanical properties of single-channel and multi-layer thin-walled parts during CMT-WAAM were studied and compared. The results show that the severe plastic deformation caused by UIT can have a positive effect on the formation of the deposition layer, microstructure, and mechanical properties. Compared with samples without UIT, the deposition specimens under ultrasonic impact current of 3 A have better macroscopic forming, with a reduced pore defect rate from 0.742% to 0.496%, a decreased average grain size of the deposition layer from 186 to 147 μm, an increased proportion of grains with sizes smaller than 50 μm from 6.3% to 15.6%, and an increased average hardness from 68.1 to 81.6 HV0.2, indicating an improvement rate of 19.8%. Moreover, they have an increased longitudinal tensile strength from 249 to 262 MPa, an increased transverse tensile strength from 264 to 278 MPa, as well as increased yield strength and elongation after fracture. The strengthening effect of UIT can reduce the sphericity of the pores in the deposition layer, effectively inducing closure and disappearance of pores in the deposition layer, promoting the transformation of the coarse columnar crystals in the deposition layer to fine equiaxed crystals, and realizing the optimization of microstructure and the improvement of mechanical properties of Al-5%Mg alloy by CMT-WAAM.