Abstract: This study presents an innovative welding solution for resolving dissimilar steel welding challenges in the construction of a high-temperature reactor in China. The welding of the steam generator (F22 steel) and main steam pipeline (P91 steel) was complicated by material property discrepancies, conflicting post-weld heat treatment (PWHT) temperature requirements, and mechanical mismatch. Through theoretical analysis and experimental validation, the following technical solutions were developed: 1) Implementing F91 transition sections and developing F22 extension segments effectively reduces direct welding risks between dissimilar steels; 2) Applying E5515-B2-V overlay welding on F91 side combined with F22 welding materials minimizes alloy composition differences. Through staged post-weld heat treatment: F91 side first undergoes 740-760℃ treatment to stabilize martensitic structure, followed by 675-705℃ treatment for F22 side. Hollomon-Jaffe parameter evaluation confirms minimal impact on F91 properties. The solution ensures compliance of tensile strength, bending performance, impact toughness, and high-temperature creep resistance with design requirements, overcoming limitations in existing welding standards and insufficient engineering experience. This breakthrough provides a safe and reliable technical approach for manufacturing critical components in high-temperature reactors, demonstrating significant engineering application value.