Abstract: The cracking position of ZG15 Cr1 Mo1 V casting was determined as angle R position of the combined valve body. The composition,microstructure and mechanical properties of the parent material around the defect were tested,and the casting was qualified for repair welding.The weldability and key points of repair welding of ZG15 Cr1 Mo1 V steel were analyzed. The repair welding of ZG15 Cr1 Mo1 V casting of medium pressure reheat combined control valve of a power plant was successfully carried out with the same material hot repair welding and the integral heat treatment. It has been running safely and stably for 4 years.