In ultrafine-grained materials and under non-equilibrium processing conditions, GBs are rarely in ground states and would instead present a multiplicity of microscopic metastable states, endowing the system with enhanced tunability. For example, recent experiments show nanocrystals can be rejuvenated by femtosecond laser and their hardness can be effectively controlled. However, a mechanistic understanding on metastable GBs’ evolution remains unclear. Here we investigate a variety of metastable GBs under fast driving conditions using atomistic simulations. Assisted with data-mining algorithm to analyze the annealing behavior of GBs at various conditions, we construct a high-fidelity energetic evolution map, showing that it can be divided into an ageing regime and a rejuvenating regime over the energy—temperature space. The ageing/rejuvenating stems from the energy imbalance during the interchanges between metastable states, and a kinetic equation is subsequently derived. The predicted energetic evolution and its implication on metastable GBs’ mechanical performance are consistent with experiments.