Abstract: The welding joint of T91/TP347H dissimilar steels fractured circumferentially on the side of T91 steel pipe after the high temperature superheater of one supercritical thermal power boiler serviced for 4×10⁴ hours. Macroscopic inspection, structure analysis, chemical composition analysis, metallographic examination and hardness testing were carried out to analyze the fracture failure reasons for the welded joint. The results show that:the thickness of the T91 steel pipe was less than the required minimum thickness after the inner wall being over turned; the change of the fluid flow caused by the structure of the turned steel pipe led to the local overtemperature, accelerating ageing and decreasing the strength; due to the insufficient strength of the heat affected zone in the side of T91 steel pipe, the pipe swelled and creeping cracks occurred; the creeping cracks continuously expanded under the effect of axial stress, and eventually caused the circumferential fracture of the steel pipe.