Based on EV charging modes of maintaining the residual energy of the EV battery and increasing battery energy for EVs, two kinds of control strategies, maintaining battery energy and implementing planning charging of EVs, are proposed for EVs to take part in primary frequency regulation. In order to hold battery State of Charge (SOC) levels and at the same time implement the frequency droop control, an elliptic function is used to construct the correlation between the battery SOC and the charging/discharging droop. According to the charging demands of EV users and the battery SOC level of EV battery, the scheduled charging power of EVs is corrected in real time to achieve the charging demands of EVs users. At last, simulations on a two-area interconnected power system show the effectiveness of the proposed control strategies.