5356 aluminum alloy has been widely applied in transportation, aerospace and other fields owing to its low density, excellent fatigue property, and superior corrosion resistance. Aluminum alloy is widely manufactured by the arc additive technique that operates at a fast manufacturing speed with simple equipment and high material utilization. The property of 5356 aluminum alloy is closely related to its microstructure. To better control the property of this alloy for the additive manufacturing of forming parts, it is necessary to study the evolution of its microstructure. In this work, 5356 aluminum alloy forming parts were produced by tungsten inert gas welding (TIG) arc additive manufacturing, and their microstructures and mechanical properties were analyzed. The 5356 aluminum alloy formed by TIG additive manufacturing was composed of α-Al matrix and β(Al₃Mg₂) phase. As the deposition height increased, the layer microstructure transformed from equiaxed grains to columnar grains and tended to stabilize at thermal equilibrium. The top layer exhibited a dendritic microstructure with serious segregation of the Mg element. The middle and lower microstructures were varied and included equiaxed grains, columnar grains, and a mixture of these, with improved Mg-element segregation. As the deposition height increased, the microhardness in the layer first decreased and then stabilized. The microhardness was larger in the interlayers than in the deposition layers. The pores gathered in the interlayers might explain the lower yield strength of the thin-walled parts than the theoretically calculated value. The tensile strength, yield strength, and elongation were all anisotropic, and the tensile property was better in the transverse than in the longitudinal direction. This result was attributable to pore accumulation between the layers of the thin-walled parts and to the uneven microstructure.