Traditional thermal power plants and hydro-power plants are difficult to deal with the problem of frequency stability caused by the rapid development of power system and the generation and parallelling in grid of renewable energy, while large-scale energy storage is applying itself in frequency regulation in power system for its characteristics of swiftness and accuracy. Energy storage system (ESS) has advantages of fast response speed, high following precision and easy-changing adjust direction, which has disadvantage of limit capacity, either. Considering the characteristics of ESS, control strategy of ESS participating in AGC is studied. Firstly, power grid AGC system model and ESS model including state-of-charging(SOC) is established. Then, with comprehensively considering the characteristics of ESS and conventional power resources, a coordination strategy between fast and slow regulation sources is proposed. Finally, three different scenarios is designed, and the efficiency of the proposed strategy is demonstrated through the sets of simulation tests.