Abstract: The weldability experiments of DH40 ship building steel were carried out by using the flux-cored wire CO₂ gas shielded multi-pass welding technology. The microstructure and mechanical properties of the joint were studied systematically and the impact toughness was discussed,especially the reason why the impact toughness at HAZ of 5 mm from fusion line of weld root decreased greatly. The results showed that the tensile strength of joints is higher than that of the base metal of 575 MPa. All the samples used in the bending test are qualified and meet the plasticity requirement. The brittleness band is easily formed at HAZ,5 mm from the fusion line at the weld root. The effect of microstructure heredity results in the coarse crystal grain,which is the secondary reason for the impact ductility decreasing, and the basic reason is the formation of a large amount of granular bainite.