With the development of vehicle-to-grid (V2G) technology, it is possible for electric vehicles (EVs) to provide active support to the power grid as flexible reserve units. Notably, the battery internal operation characteristics largely impact the EV’s active power grid support capability, which should be carefully considered. Therefore, this paper analyzes the internal operation characteristics of the EV battery, i.e., the relationship between the peak power and state of charge (SOC). With the consideration of the charging demand of EV owners, this paper proposes an operation area evaluation method for the EVs when providing active support to the power grid. This method can ensure that the EVs provide stable active support to the power grid while meeting the charging demand of EV owners. The EV operation area is validated by simulation.