Abstract: The 2 mm thick Al-Mg alloy plate is welded by vacuum electron beam welding with ER5183 filler wire, and the microstructure and mechanical property of the welded joint are analyzed and tested. The results show that, with the suitable welding technique, the Al-Mg alloy joint with good appearance of weld can be obtained. Microstructure analysis shows that the fusion zone (FZ) is composed of columnar crystal and equiaxed dendrite, which are mainly α(Al) matrix and β(Al₃Mg₂) strengthening phases. There exist lots of entangled dislocations and secondary phase particles in the weld line. The hardness in FZ decreases to a certain extent compared with that of the base metal (BM). The tensile strength of the joint by the optimized welding parameters is 311.2 MPa, which is 96.9% of that of the BM. There are many dimples on the tensile fracture surface of the joint, and the ductile feature is obvious.