ZG15 Cr2 Mo1 is the common material used for large castings in thermal power plants. The welding repair technology of ZG15 Cr2 Mo1 main steam combined regulating valve in the thermal power plant is introduced,including the removal of defects,welding preparation,heat treatment procedure,welding technology and inspection. The main steam combined regulating valve is repaired by manual metal arc welding with ENi Cr Fe-2 electrode,single U groove and preheating temperature of 100 ℃～150 ℃. To get a good result,the manual metal arc welding with low current is used for the backing weld,and the multi-layer and multi-pass welding is also used for continuous welding. After welding,the penetration test and hardness test are carried out,no effect is found,and the hardness test results also meet the standard requirements,the results show that the technology can repair the large castings of ZG15 Cr2 Mo1 in thermal power plants.