Large Nuclear power steam turbine Low pressure (LP) rotor composed of several shafts and discs with 25Cr2Ni2MoV steel was manufactured by gas tungsten-arc welding (GTAW) for backing welding and submerged-arc welding (SAW) for filling welding. In this study, the welding joint is investigated and it is found that the main microstructure of weld metal and base metal is typically bainite, while tempered martensite is observed in heat affected zone (HAZ) by over-quenching imposed by welding cycling and post weld heat treatment (PWHT). Compared with the weld metal of SAW, the microstructure of GTAW is relatively fine and homogeneous, due to its lower heat input. The hardness test indicates that the maximum hardness of HV400 occurs in HAZ because of tempered martensite. Weld metal of SAW has the hardness of HV270, which is a little bit lower than GTAW’s on account of fine grain. Based on the researches, the trial welded rotor and production rotor have been successfully produced, relevant inspections and tests were conducted, the results are well consistently with original research.