Abstract: Deep-penetration TIG welding of 1561 aluminum alloy was conducted, and the microstructure and mechanical properties of the joints were analyzed systematically. The results show that the pores and cracks are not found in welds. The coarse grain zone with shape of "∧" is observed in the weld cross section of 1561 aluminum alloy. There are fine grain zones on the top and bottom zones of the coarse grain zone. After the cosmetic welding, the grains of backing weld grow up obviously, the remelting occurs in the grain boundaries of the fusion zone, and the softening phenomena still occurs in the joint. The microhardness of weld and heat affected zone is lower than that of the base metal, but there is no obvious softening phenomenon in the heat affected zone. Moreover, the tensile strength of the joints was about 314 MPa, which shows that the deep-penetration welding is suitable for the 1561 aluminum alloy with high magnesium.