As energy storage device，the DC side capacitor is required for the normal operation of the parallel active power filter （PAPF），which directly affects the harmonic compensation effect of PAPF.Aiming at the problem of the relationship between DC side capacitor of single-phase PAPF and voltage fluctuation，an approach is presented according the analysis of power components which have influence on the fluctuation of DC voltage through the instantaneous power flow.And the capacitor energy fluctuation precision value is obtained by integrating the power components under the specific load conditions.The capacitance under a certain compensation capacity and capacitor voltage fluctuation amplitude value are derived.Simulation and experiment results confirm the validity and feasibility of the method.