Abstract: The low-activated martensitic steel was subjected to vacuum diffusion welding test under different welding parameters. The microstructure of the weld zone was observed by optical microscope (OM) and scanning electron microscopy (SEM), and the mechanical properties of the welds were tested. The obtained microstructure of the low-activated martensitic steel welds is mainly composed of lath martensite and a small amount of retained austenite and exhibits preferable bonding effect. Increasing the welding temperature and prolonging the holding time in a certain range can improve the tensile strength of the joints. It also promotes the coarsening of austenite grain, which is detrimental to the tensile strength and impact toughness. The tensile strength of the joints after post weld heat treatment (normalizing + tempering) is lower than the as-welded one, but the microstructure stability and impact toughness of the welds are obviously improved.