System frequency control is difficult in power grids with high proportion of renewable energy, and the secondary frequency regulation of the power system faces new challenges. Therefore, an optimal control strategy of thermal power units with electric vehicles (EV) participating in the auxiliary service market is proposed. In the optimization stage of frequency regulation command distribution ratio, this paper considers the frequency modulation cost and the compensation cost of batteries. With the goal of maximizing the revenue of frequency modulation agents, a rolling optimization control strategy is proposed. In the response stage of the EV charging station, this paper considers the state of charge and adjustable capacity of each EV in the station. Then a dynamic adjustment of charging and discharging power control method with zones and priorities is adopted to respond to the frequency regulation requirement. Finally, a two-region interconnected power system model with EV power stations is constructed to simulate and analyze the performance and economy of this strategy. The results show that the strategy proposed greatly reduces frequency response bias and improves the economic benefits of frequency modulation agents.