The microstructure evolution and variation of mechanical properties of nickel-based U720Li superalloy were investigated during long-term aging (3000 h) at 680, 700, and 730 °C. The changes in size and morphology of γ′ phase were determined. Results show that the coarsening behavior of γ′ phase is controlled by the diffusion process. The tensile and creep properties of U720Li superalloy remain stable during the long-term aging at 680 and 700 °C. However, the yield strength gradually decreases and the ductility increases after long-term aging at 730 °C for 500 h. The fatigue and creep properties of U720Li superalloy decreases sharply after long-term aging at 730 °C, which is mainly related to the morphology evolution of γ′ phase.