Abstract: To study the strengthening effect and mechanism of Er element on the weld, the remelting welding of Al-Mg alloy with Er was adopted, and the organizational structure and mechanical properties of the joint were observed. The results show that there is primary Al₃Er phase precipitation on the grain boundaries in weld, but the quantity of it is fewer than that of in the base material, and the precipitation doesn't form a continuous ring. The secondary Al₃Er precipitation also can be found in the welding, but the precipitation phase quantity is very few. Er elements will presence in the solid solution or segregation way. The strength of Er element on the weld joint is mainly based on the primary Al₃Er to refine the grain. The effect of the secondary phase on the weld joint is small, and so the weld joint mechanical properties decrease evidence. The tensile strength of welded joints is 297 MPa, and the strength coefficient of welded joints is 72% of base material. Tensile fracture position is mainly in the center of the weld and near the fusion zone.