Electric vehicle industry develops rapidly, so charging loads connected to grids will have a significant impact on power systems due to uncertainty of charging behavior of electric vehicles. On the other hand, as a movable and adjustable energy storage unit, electric vehicles can provide energy storage auxiliary services for a power system including peak regulation. However, the potential of electric vehicle suppling energy storage auxiliary services needs to be evaluated. This paper analyzes the feasibility of electric vehicles participating in grid auxiliary services, calculates the auxiliary service capacity of different types of electric vehicles, and then calculates the auxiliary service capacity of electric vehicle cluster. And an example of Nanjing is proposed to calculate the potential capacity of this cluster.