As an important part of smart grid, electric vehicles (EVs) can supply reservation to grid by regulating charging and discharging power, providing ancillary services such as peak regulation, frequency regulation, etc. But the capacity is influenced by user demand, battery loss, etc. Firstly, the control model of EV involving in power dispatching is analyzed. Then an evaluation algorithm of EV real-time schedulable capacity is proposed. The algorithm considers the user demand, battery life and battery power constraints comprehensively. Finally, a scene of smart charging to stabilize the total load fluctuation is simulated, and the proposed algorithm is applied to calculate EV aggregator real-time schedulable capacity used for primary frequency regulation, secondary frequency regulation, and tertiary frequency regulation, based on required duration of schedulable capacity, which verifies the effectiveness of the algorithm.