A nuclear power plant has completed its first overhaul metal supervision in 2024, and the metal supervision found cracks in the welds of the water chamber partition of the 1B high-pressure heater. These cracks were repaired by the fabricator during the overhaul, but the new weld was tested for liquid penetration and there were still more cracks on the surface than before the repair. Faced with this problem, the team identified the cause of the re-cracking of the repair weld by analysing the morphological characteristics of the crack and investigating the manufacturing process of the high-pressure heater, and removed and re-repaired the crack. There are actually two dissimilar metal welds at the cracking position, involving carbon steel, low-alloy steel, and stainless steel, and repairing one of the welds may crack the other weld, and the weld is inside the equipment, and the space is narrow, and the repair process involves pre-welding preheating and post-welding heat treatment, and the repair process is complex. To solve the above problems, the team developed a detailed rework plan and introduced a post-thermal process to balance the interaction between the two dissimilar metal welds, and finally achieved a first-time success in the repair. This paper briefly introduces the connection structure, the causes of weld cracking and the final repair process of the water chamber partition of the high-pressure heater, aiming to provide reference experience for the subsequent welding problems of such complex materials.