Abstract: The electron beam welding technique was used to weld the 9 mm-thick 15 CrNi3MoV high strong low alloy steel. The welded joint with fine appearance and no internal defects was obtained with proper welding parameters, and its welding coefficient reached to 0.98 after the heat treatment. The microstructure and mechanical properties of the welded joint were studied.The results show that the welded joint mainly consists of bainites, while sorbites are found in the welding seam and the coarse grain zone and the granular bainites are observed in the fine gain zone; the impact ductility is improved compared with that of the base metal; after modification welding, the columnar crystal formed in the first welding process is destroyed, which can improve the microhardness of the welding seam.