This research focused on two candidate materials (alloy A and alloy B) for superheater and reheater in high-parameter advanced ultra-supercritical power plants. It systematically analyzed the phenomenon of local grain boundary widening, the kinetics, and the mechanism of grain boundary widening of two kinds of Ni-based superalloys during long-term aging. The results show that the main precipitates of two alloys in the widening grain boundary region are M₂₃C₆ carbide and grain boundary γ′ phase during high-temperature and long-term aging. The evolution of grain boundary widening of two alloys with aging time follows the JMAK equation. The formation process of grain boundary widening consists of three stages. In the first stage, the M₂₃C₆ carbides near the grain boundaries are treated by meltback, inducing the coarsening of M₂₃C₆ carbides at grain boundary and the grain boundary migration. In the second stage, new M₂₃C₆ carbides are precipitated after grain boundary migrating, which makes M₂₃C₆ carbides in the grain boundary area arrange in multiple layers, and the width of grain boundary increase. In the third stage, the γ′ phase will precipitate at the M₂₃C₆/γ interface with the decreased coarsening rate of carbides, and the short-circuit diffusion of grain boundary makes the γ′ phase grow and the width of grain boundary further increase.