Abstract: The inertia level is a key factor affecting the frequency characteristics of power systems, and accurate inertia assessment is essential for maintaining grid frequency stability. With the increasing penetration of battery energy storage systems (BESS), their contribution to inertia support becomes increasingly critical. However, frequent charge–discharge cycles accelerate battery aging, thereby weakening their inertia-supporting capability. To address this issue, this paper proposes an inertia stability region (ISR) evaluation method that considers the long-term aging effect of BESS to prevent overestimation of system inertia and avoid potential frequency instability. First, a peak power degradation model is established for inertia-supporting scenarios to analyze the long-term degradation characteristics caused by cycling. Then, based on the grid frequency response model, the ISR upper boundary is updated according to the power degradation trend, while the lower boundary is determined using load forecast results. Finally, a complete ISR is formed, and an inertia stability margin index is proposed. Simulation results demonstrate that the proposed method effectively captures the impact of battery aging on grid inertia levels and provides a reliable basis for inertia scheduling.