Energy storage is an important component of grid flexible dispatch resources. In addition to responding to the urgent needs of peak shifting and valley filling, the capacity of centralized energy storage (CES) can also exert huge grid backup capacity under the high penetration of new energy. However, under the dual influence of power generation (consumption)plan and its own power constraints, this capability exhibits obvious dynamic characteristics, which brings certain difficulties to accurate assessment. Therefore, a concept of operational viability domain is introduced. Its reserve capacity is analyzed by describing the electricity and power boundaries of CES, a new method is proposed to calculate the reserve capacity of CES operation under a given power generation (consumption) plan. Based on the simulation analysis of an energy storage power station and a provincial power grid, the effectiveness of the method is verified. The results show that taking into account the reserve capacity of energy storage can effectively reduce the abandonment of the system. Furthermore, the possible effects of energy storage electricity capacity ratio, reserve contracts, etc. on dynamic reserve capacity are quantitatively analyzed. The suggestions for further optimization of energy reserve capacity are given.