Abstract: Multi-layer and multi-pass arc welding was conducted on 0Cr13Ni4Mo martensitic stainless steel plates under different welding heat inputs (9, 13, 17 kJ · cm⁻¹), and the effect of welding heat input on the microstructure and mechanical properties of the welded joints was studied. The result show that the microstructures of welded joints under different welding heat inputs were all composed of lath martensite, a small amount of δ ferrite and residual austenite. With the increase of welding heat input, the lath martensitic structure of the weld and the heat-affected zone became coarse, and the δ ferrite content increased. Under different welding heat inputs, the tensile strength and yield strength of the welded joints were approximately 810, 600 MPa, respectively, which were higher than those of the base metal and complied with the project specification. The yield ratio was less than 0.9. The joints had good tensile properties, and fractured at the base metal after tension. With the increase of welding heat input, the impact absorption energy decreased. When the welding heat inputs were 9, 13 kJ · cm⁻¹, the size of the dimples in the impact fracture was slightly larger and more uniform, indicating the better impact toughness of the joint. The hardness of the weld under different welding heat inputs was 310–340 HV, and the average hardness was higher than that of the heat affected zone and base metal. With increasing welding heat input, the hardness of the weld and the heat affected zone slightly decreased.