<p>1Cr10Mo1NiWVNbN martensitic heat-resistant steel (12Cr) is a crucial material for essential components in thermal power plants. An electron beam welding experiment was conducted on 20-mm-thick 1Cr10Mo1NiWVNbN steel. The effects of the welding beam on the microstructure, phase composition, and mechanical properties of the joint were systematically studied. The results show that the weld and heat-affected zone are composed of a large number of martensitic structures, and the size of the lath is smaller than that of the base material. The increase in welding beam current will affect the size of the martensitic laths and coarsen the structure of the weld and the heat-affected zone. According to Kurdjumov–Sachs orientation relation, the martensitic lath was reconstructed into proto-austenitic grain (PAG). The welded joint exhibited a weak cubic texture, with the texture strength reaching the maximum in {110} direction. After welding, the hardness of the weld significantly increased, and the tensile strength was enhanced due to the dissolution of ferrite. The strengthening of the weld was attributed to grain refinement and precipitation. The tensile specimen fractured in the base material, with the tensile fracture mode primarily being ductile.</p>