Abstract: In order to explore the effect of service on the mechanical behavior of T91/TP347H dissimilar steel welded joints，the high temperature creep experiments of T91/TP347H dissimilar steel welded joints were carried out at 620 ℃ under different stress levels, and the creep behavior was analyzed. The fracture morphology, microstructure and microhardness of T91/TP347H dissimilar steel welded joints before and after service were studied by scanning electron microscope, optical microscope and microhardness tester. The results show that: under high stress level, the welded joint fails faster, and the fracture is rougher and has large dimples; compared with the original state, the T91 side of the welded joint after servicing 1×105 h has coarsened precipitates, and the grain boundary is increased and becomes more wider, easy to produce stress concentration phenomenon, which makes the material prone to brittle fracture along the grain boundary; after service, the T91 side appears softening, while the TP347H side and the weld are strengthened, the hardness of the weld increases significantly, and the average hardness of the T91 side of the welded joint is larger than that of the TP347H side. In this paper, the changes of T91/TP347H dissimilar steel welded joints before and after service are analyzed through macroscopic mechanical experiment, microstructure observation and microhardness test.Highlights: (1) The creep performance and influence of service on the BM (base metal), HAZ (heat affected zone) and welding of T91/TP347H dissimilar steel welded Joints are investigated based on the multi-scope analysis such as macro-mechanical experiments, microstructure observations and fracture morphology.(2) Based on the microscopic hardness test, the hardness change of the BM and welding of T91/TP347H DSWJ before and after service are studied. It is considered that servicing process can harden the welding and HAZ of TP347 steel.