Abstract: The dissimilar steel welded joint of platen superheater of a subcritical power plant boiler fractured during operation. The fracture causes of the dissimilar steel welded joint were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination and microscopic analysis. The results show that the difference of creep strength between the base metal on both sides of the platen superheater joint was large, which caused the early failure of the dissimilar steel welded joint. At the same time, the welding quality and forming were poor, and the welding joint was affected by expansion thermal stress, welding residual stress and temperature fluctuation thermal fatigue accelerated the process, which eventually led to the fracture of the dissimilar steel welded joint.