Abstract: The reheater/superheater tube sections of T91 steel after long-term service （6.2×10⁴‒12×10⁴  h） at high temperature of 541‒550 ℃ was taken as the research object. The microstructure and mechanical properties of T91 steel after long-term service were investigated through multi-scale microstructural characterization and quantitative statistics of precipitates. The results show that after long-term service, the martensitic laths in T91 steel became widened, the dislocation density within the laths decreased, the subgrain size increased, and Laves phases were precipitated at grain boundaries. After long-term service, the grain boundary precipitates （mainly M₂₃C₆ and Laves phases） exhibited a chain-like distribution and were coarsened. The content of key alloying elements in M₂₃C₆ and Laves phases showed a positive correlation with the extension of service time. The strength and hardness of T91 steel decreased after long-term service. The steel with a relatively shorter service time （6.2×10⁴ h） exhibited the lowest strength and hardness due to its largest subgrain size and grain boundary precipitate size.