Abstract: The welding seam of the holding pipe of the medium temperature reheater of a 1025t/h boiler in a power plant was analyzed by macroscopic inspection, material composition analysis, metallographic examination and energy spectrum electron microscope analysis. It is concluded that, due to material misapplication, the dissimilar steel butt weld bears high stress and low cycle fatigue stress, and the weld root has poor forming, the dimension has local sudden change, which casues local over-temperature of the pipe section to form decarbonization zone and intercrystalline cracks. The intercrystalline cracks initiate at the fusion line of the weld root and expand to the outer wall, eventually forms the penetrating crack, resulting in leakage.