Output of PV generation system features large fluctuation and strong stochastic effect. Battery energy storage system (BESS) is integrated with large-scale PV system to mitigate the power shock to the grid by smoothing out the output power fluctuation. This paper first presents a method to evaluate the capability of different energy storage control strategies to smooth out PV power fluctuations. Then an energy storage control strategy based on state of charge (SOC) adjustment is brought out. In this approach, fluctuation components of PV output power in relatively high frequency domain is compensated by tuning the BESS output power. When the SOC of battery is too high or too low, the BESS output power is adjusted. Thus, the SOC of battery stays in safe zone while the effect of compensation is not deteriorated. Simulation has proved that the method works well even when the volatility of PV output is rather strong, and it does not need a large battery capacity.