Abstract: Electroslag welding involves high heat input, which may cause material hardening in the weld and heat-affected zone. At the same time, the electroslag weld is in an inaccessible state, making the welding quality difficult to control. Electroslag welded joints of Q355B steel were designed and fabricated. Through metallographic analysis, it is found that the fusion line and heat-affected zone of electroslag welding have large widths, and obvious welding defects exist in the weld metal. Monotonic tensile tests on the weld metal and heat-affected zone materials of electroslag welding were conducted, and the mechanical properties and constitutive relationships of the materials were determined. Notched round bar specimens for the weld metal and heat-affected zone materials of electroslag welding were fabricated, and monotonic tensile tests were conducted. Combined with finite element simulation of the tests, the parameters of the void growth model for the materials were calibrated, and the ability of the weld and heat-affected zone materials of electroslag welding to resist ductile fracture was evaluated.