Abstract: This paper mainly analyzes the microstructure and anisotropic mechanical properties of ER4220 aluminum alloy fabricated by CMT arc additive manufacturing. The research results show that the inter-layer and intra-layer microstructure patterns of the CMT arc additive manufacturing ER4220 aluminum alloy exhibit significant differences. Along the deposition direction, it has a fine dendritic substructure, and the dendrite morphology changes little at different positions. In the inter-laminar bonding area, the amount of Al-Si peritectic phase precipitated between dendrites is relatively small and is discontinuously distributed, but the size is large. While the amount of Al-Si peritectic phase with a cluster-like precipitated within the layer is large, continuously distributed between dendrites. The microhardness at the bottom of the arc additive manufacturing aluminum alloy specimen is approximately 72.5 HV. With the increase in the number of deposition layers, the microhardness slightly increases, and then with the increase in deposition height, the microhardness does not change much. The microhardness fluctuates between 65-85 HV along the deposition direction. The tensile properties of the CMT arc additive manufacturing ER4220 aluminum alloy have a strong anisotropic property. The tensile strength in the horizontal direction is 213.63 MPa, with an elongation of 9.77%, while the tensile strength in the vertical direction is 176.59 MPa, with an elongation of 1.19%. The horizontal direction has better tensile strength. The fracture mechanisms of the tensile specimens in different directions are all ductile fractures.