Due to the requirement of leveling off the output power fluctuations, the relationship between the battery energy capacity and PV power fluctuation is analyzed by adjusting the filter time constant according to the state of charge (SOC) based on the low-pass filtering principle, and the optimization design method of battery power capacity and energy capacity is put forward. With the reasonable battery configuration for PV power system, the battery over-charge and over-discharge can be avoided, and the best operation condition and long service life would be obtained. A simulation Matlab model is tested to verify the feasibility of this method.