The modular High-Temperature Gas-Cooled Reactor nuclear power plant has inherent safety, simple system, high power generation efficiency and wide use. Incoloy800H and 2.25Cr-1Mo heat exchange tubes are used for High-Temperature Gas-Cooled Reactor (HTR-PM) Steam Generator. The steam generator uses the Incoloy800H and 2.25Cr-1Mo heat exchange tubes to transfer the high temperature helium medium to the water medium. The dissimilar steel welds are designed between Incoloy800H and 2.25Cr-1Mo heat exchange tubes. The heat exchange tubes of HTR-PM Steam Generator with small OD, large thickness, and design structure complicated. The dissimilar steel welds between two kinds of heat exchange tubes with limited welding operation space and welding seam quality requirements high. During the HTR-PM Steam Generator operation process, Incoloy800H heat exchange tube, 2.25Cr-1Mo heat exchange tube and the dissimilar steel welds between Incoloy800H and 2.25Cr-1Mo heat exchange tube are service in high temperature environment for a long time, until the steam generator fails. The surface forming quality, conventional properties and high temperature stress-rupture of dissimilar steels weld is required. The quality of dissimilar steel weld of Incoloy800H and 2.25Cr-1Mo heat exchange tubes is directly related to the safe operation of steam generator. Through systematic research including tube-to-tube joint type, the welding material, welding parameters, heat treatment process etc. The results show that using appropriate GTAW process with Nickel base alloy consumable inserts, welded joints can meet all design requirements such as strict inner and outside surface forming quality, conventional performance and the strength of stress-rupture test in HTR-PM Steam Generator service environment. This process has been used in the product manufacturing of demonstration projects, and provides good demonstration experience for similar application scenarios in the industry.