Due to its uncertainty and other characteristics, the high proportion of renewable energy has brought great challenges to the stable operation of power system. A more rapid and stable power system regulation capability requires the participation of multiple flexible resources. For this purpose, an Overall System Flexibility (OSF) index based on scheduling level is proposed. Firstly, the flexibility characteristics of net load, unit, energy storage and tie line of the system are analyzed and their flexibility model is established. Then, a deterministic index based on capability factor is proposed, which can describe the flexibility of the system as a whole. And the dispatching model of power system considering regional union is established and the indexes are calculated. Based on the three-area IEEE RTS96 system, 10 units of the system are selected. The OSF index is calculated and compared from the aspects of energy storage, tie line and reserve capacity. The results show that the proposed index can directly reflect the flexibility of the whole system. This work is supported by National Natural Science Foundation of China (No. 51877154).