Abstract: The fillet welding seam of the 660 MW high-pressure heater nozzle cracked and the cracks extended to the base material of the cylinder in the process of welding. The weld seam was examined by macrographic examination, hardness testing, metallographic examination, scanning electron microscope and energy spectrum analysis to analyze the cracking reasons. The results show that:the formation was bad at the fillet weld root of the nozzle and cylinder, and there were welding defects such as slag, porosity and so on; under the action of welding stress and structural restraint stress, stress concentration generated on the position of welding defects, which led to crack initiation and propagation. The integral forging scheme which matched with the repairing gap of the cylinder was adopted, and the forging strength calculation could meet the design requirements and the repair had been successfully done at the first try.