The efects of long-term aging at 700 and 750 °C on microstructure and mechanical properties of a new developed tungsten bearing heat-resistant alloy used for advanced ultra-supercritical power plant was investigated both experimentally and thermodynamically. Experimental results showed that the mechanical properties maintained excellent stability after long-term aging at 700 °C for 10,000 h, while the impact absorbing energy decreased sharply after 1000-h aging and then kept constant till 10,000 h. The main precipitates after long-term aging at 700 and 750 °C were M23C6, MC and homogeneous γ′-phases. The mass fraction of M23C6 carbides increased with increasing aging time, and M23C6 carbides precipitated in shape of chains and lamellas on grain boundaries. The slight decrease in MC carbides during aging may be due to degradation reaction. The weight fraction of γ′-phase increased with the aging time, and then changed little after 5000 h; γ′-phase exhibited excellent microstructure stability and low coarsening rate during long-term aging at 700 °C. However, the coarsening rate of γ′-phase was much higher at 750 °C.