Abstract: Microstructure analysis, room-temperature and high-temperature mechanical properties tests and hightemperature creep tests were conducted on 10 CrMo910 steel used in the main steam pipeline of a thermal power plant unit that had been in operation for 35 years. These tests aimed to investigate the steel’s performance changes and residual life after extended service. The results indicate that the mechanical properties of the main steam pipeline after extended service have generally decreased by more than 30% compared to new materials. Additionally, the material has undergone spheroidization of grade 3.5, with dynamic redistribution of alloy elements and notable precipitation of second-phase particles. The creep performance of the main steam pipeline after extended service has declined by more than 33.08% and 85.96% compared with the same materials for 25 years and 15 years of service, respectively. Under operating conditions, the remaining life of the pipeline is 42967 h, indicating a significant decline in the creep properties of the material.