Output fluctuation of intermittent sources can be mitigated by coordinating with ESS (energy storage system) so that output stability can be improved. This paper proposes a real time control strategy for ESS based on sliding least square algorithm and SOC (state of charge). Output power can be determined by sliding least square algorithm. Combined with regulation of SOC and maximum fluctuation power limit, discharge depth of ESS can be effectively reduced during the smoothing control process. The simulation results show that compared with traditional filter algorithm based control strategy, the proposed control strategy can achieve less SOC fluctuation, largely reduce required ESS energy capacity and lengthen battery lifecycle with identical smoothing effectiveness.