The microstructure, macro morphology, oxide layer morphology and mechanical properties of T91 steel after 96000 hours of service at high temperature and high pressure were investigated. The results show that the service steel pipe has good macroscopic appearance, no obvious wear, scratch, bulge, deformation, surface crack and other abnormal features, and no obvious thinning of wall thickness. The chemical composition of different pipe sections meets the standard, and the microstructure is mainly tempered martensite, which is evenly distributed, with a small amount of massive ferrite and no obvious coarse phase. The oxidation products are divided into internal oxide layer and external oxide layer, and the internal oxide layer is dense and continuous, microcracks appear in the loose oxide layer. After 96000 hours of service, T91 steel has good mechanical properties. The tensile properties at room temperature and high temperature are good. For room temperature, the yield strength and tensile strength are 480MPa and 630MPa, respectively. Under 500°C and high temperature, the yield strength exceeds 319MPa, the tensile strength exceeds 380MPa, and the elongation exceeds 17%. Hardness test values are between 185∼250 HBW.