Abstract: In view of the large-scale shared energy storage in areas with dense renewable energy power stations, this research studies the optimal operation strategies of the battery energy storage system (BESS). The main purpose of the optimization strategies is to make the best use of BESS' regulation capability to participate in the ancillary service market, for instance, to provide secondary frequency regulation, under the premise of fulfilling the obligations of primary frequency control for multiple wind and/or solar power stations. With the goal of maximizing the revenue of the shared BESS, an optimal bidding strategy for providing secondary frequency regulation service considering the base charging/discharging power schedule has been proposed based on the rules of the ancillary service market, while the energy storage degradation cost, maintenance cost and frequency regulation revenue are taken into account. The bidding strategy optimization model is then converted into a mixed integer linear programming problem, which can be solved by the off-the-shelf software. The simulation results based on the history data indicate that the proposed method effectively makes use of the energy and power capacities of the BESS. Moreover, the BESS has the ability to provide the secondary frequency regulation service and price arbitrage, as well as taking the primary frequency regulation obligations of the renewable energy power stations.