Abstract: Hardness and tensile property tests were conducted on samples of T91/Super304H dissimilar steel welded joints from the secondary superheater tubes of a 1000 MW ultra-supercritical power unit in a domestic power plant, which had been in long-term service for 120,000 hours. Analysis methods such as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to investigate the microstructural aging behavior of the dissimilar steel welded joints during long-term service. A comparison was also made between the microstructure and properties of the original dissimilar steel welded joints. The results indicated that cracks in the T91/Super304H dissimilar steel welded joints of the secondary superheater tubes often occurred at the fusion line on the T91 steel side. This was due to the aggregation and growth of Cr₂₃C₆ at the grain boundaries and coarser of Nb(Ti)C particles on the T91 steel side during long-term service. This led to a significant increase in the hardness of the T91 side fusion line of the T91/Super304H dissimilar steel welded joint, which subsequently caused cracks to initiate, propagate, and ultimately fracture on the T91 steel side fusion line.