In this study, precipitates in COST-FB2 steel equilibrium conditions were calculated using Thermal-Calc thermodynamic software. The microstructure, M23C6 carbides, and Laves phase evolution of COST-FB2 steel for the rotors of the supercritical power station at 620 ℃ under different aging times were studied by scanning and transmission electron microscope; and chemical phase analysis. The effects of the microstructure and precipitates on the properties of the steel were also analiyzed. The results showed that the room temperature strength and plasticity of COST-FB2 steel changed minimally after aging, whereas the strength and plasticity at high temperature fluctuated. The impact property and hardness decreased greatly after 1 000 h of aging, but with the further extension of time decreased slightly. The martensitic lath structure of COST-FB2 steel was stable during 0-10 000 h of aging, the dislocation density and small-angle boundaries decreased, and the average particle size of M23C6 carbides increased. The Laves phase began to precipitate at 2 000 h of aging, and increased in size from 2 000 h to 10 000 h. After 10 000 h, the average diameter was approximately 410 nm, and the coarsening rate of the Laves phase was much greater than that of the M23C6 carbides. After aging for 2 000-6 000 h, the number of Laves phase in the unit area increased continuously, but after 6 000 h, the number begans to decrease; after 8 000 h, it tended to be in equilibrium. The rotor forgings of domestic COST-FB2 steel exhibited a stable microstructure and properties after aging at 620 ℃ for 10 000 h.