Abstract: This study used the layer-by-layer UIT process to reinforce the CMT WAAM of Al-5%Mg alloy thin-walled parts. The effects of different ultrasonic impact currents on macroscopic forming, pore evolution, solidification microstructure, and mechanical properties of single-layer and multi-layer thin-walled parts in CMT-WAAM were studied and compared.The results showed that the severe plastic deformation caused by ultrasonic impact treatment 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 3A had 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 μm to 147 μm, an increased proportion of grains with sizes smaller than 50 μm from 6.3% to 15.6%, an increased average hardness from 68.1HV0.2 to 81.6 HV0.2, with an improvement rate of 19.8%, an increased longitudinal tensile strength from 249 MPa to 262 MPa, an increased transverse tensile strength from 264 MPa to 278 MPa, as well as increased yield strength and elongation after fracture.The strengthening effect of ultrasonic impact treatment 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 CMT-WAAM Al-5%Mg alloy.