The invention relates to a welding repair method for large-sized heat-resistant steel castings, and solves the problems that the welding stress is difficultly removed, the defects of incomplete fusion, incomplete penetration, delayed crack and slag inclusion are easily caused in a repair process and the repair quality is difficultly guaranteed as the heat-resistant steel castings are poorer in welding performance, complicated in defect position and large in workpiece in an existing manual arc welding type surfacing welding mode for repairing large-sized castings. The method mainly comprises the following steps: 1, removing the defects; 2, performing heat treatment; 3, performing dye check; 4, performing statistics on the repair welding quantity; 5, preheating before welding; 6, performing repair welding on defect parts; 7, cleaning and repairing; 8, performing post-welding heat treatment; 9, performing nondestructive inspection. According to the method, the large-area defects of the castings can be effectively repaired, the repair quality is guaranteed, the processing and use requirements of the castings can be met after repair, and the economic loss of scrapped important or major castings is reduced. The method is used for repairing the large-area defects of the large-sized heat-resistant steel castings.