Abstract: The welding of 1.8 mm thick 1 000 MPa grade martensitic steel was carried out using the method of melting electrode gas shielded welding. The influence of welding heat input on the hardness distribution, microstructure evolution, and mechanical properties of welded joints was systematically studied. The results show that compared with the base metal, the hardness of the weld and heat affected zone underwent varying degrees of softening, with the softening phenomenon in the fine-grained zone being the most significant. As the welding heat input increased, the strength and plasticity of the welded joint decreased, while the toughness increased. The hardness of the weld zone, critical zone, and subcritical zone slightly decreased, while the hardness of the coarse-grained and fine-grained zones slightly increased. There was no significant change in the impact performance of the weld and heat affected zone, and the fracture position of the joint changed from fine-grained zone to subcritical zone.