Abstract: In response to the difficulty of selecting on-site heat treatment processes for T91/TP347H dissimilar steel welding joints in thermal power plant boilers, this study investigates the effects of three tempering temperatures of 700 ℃, 720 ℃, and 740 ℃ on joint performance, aiming to provide direct and reliable process basis for on-site engineering applications. The results indicate that tempering temperature can significantly regulate the mechanical property gradient of the joint: when the temperature rises to 740 ℃, the yield strength of the heat affected zone on the TP347H side drops sharply to 240MPa due to severe sensitization, and the elongation at break deteriorates to 12%, clearly defining the high risk of this temperature in field applications; On the T91 side, there is always a hardness peak of 260HV in the heat affected zone. This study innovatively combines precise experimental measurements with finite element creep simulation to construct a full chain evaluation system from "process organization performance" to "service behavior". Simulation predictions indicate that the creep damage core area will migrate towards the T91 fusion line during long-term service, and bulging deformation will occur 10 years later. This study proposes for the first time a field tempering process window (700-720 ℃) that balances performance and creep life, providing key data support and theoretical guidance for ensuring the safety and reliability of welding structures in thermal power plant boilers and optimizing operation and maintenance strategies.