In order to accurately evaluate the creep-fatigue lifetime of GH720Li superalloy, a lifetime prediction model was established, reflecting the interaction between creep damage and low-cycle fatigue damage. The creep-fatigue lifetime prediction results of GH720Li superalloy with an average grain size of 17.3 μm were essentially within a scatter band of 2 times, indicating a strong agreement between the predicted lifetimes and experimental data. Then, considering that the grain size of the dual-property turbine disc decreases from the rim to the center, a grain-size-sensitive lifetime prediction model for creep-fatigue was established by introducing the ratio of grain boundary area. The improved model overcame the limitation of most traditional prediction methods, which failed to reflect the relationship between grain size and creep-fatigue lifetime.