Abstract: In order to further explore the weld properties of ULCB steel, butt joints of 14 mm steel were welded by vacuum electron beam penetration welding under different beam intensity parameters. By comparing the weld profile, it was found that the best weld joint shape was formed at a beam intensity of 100 mA. Meanwhile, tensile properties, hardness, impact test and microstructure were conducted on the joint at a beam intensity of 100 mA. The results shown that the fracture zone of the tensile specimens was in the base metal zone, the tensile strength was 761 MPa, the yield strength was 669 MPa, and the hardness was range 270~330 HV. The fracture area of impact specimen was in heat affected zone, the average impact energy in weld zone was 288 J, and the average impact energy in heat affected zone was 273 J. It is shown that α' martensite phase occurs in the weld zone and heat affected zone, which resulted in phase transformation strengthening in weld zone and heat affected zone. The strength and hardness of welded joints were higher than those of base metal.