A high temperature T91 + TP347HFG dissimilar steel welded Joint of 350 MW supercritical circulating fluidized bed unit was found to crack and leak.The specimens were investigated by means of macroscopic examination,chemical composition analysis,mechanical property test,hardness test and metallographic structure observation,etc.The test results show that the leakage of the dissimilar steel welded joint is due to the poor welding quality and the post-weld heat treatment work is not carried out according to the requirements.