When a three-phase four-wire unified power quality conditioner（UPQC） works under unbalanced and nonlinear conditions， the voltage fluctuations on the DC side are inevitable. These fluctuations will directly affect the series and parallel converters of UPQC transforming normally. The causes and the inevitability of fluctuation of voltage on the DC side are discussed in detail. In order to eliminate the negative effect of voltage fluctuations on the series and parallel converters， the concept of DC voltage regulation coefficient is proposed. It is deduced from the principle of sinusoidal pulse width modulation（SPWM） which is then used as a suppression measure of voltage fluctuation on DC side in the direct current control strategy with load voltage feed-forward for parallel converter and the double-closed-loop control strategy for series converter. Experimental results show that using these control strategies， the impact of voltage fluctuations on DC side on the series and parallel converters can be inhibited. Compared with the control strategy without the DC voltage regulation coefficient， UPQC compensation performance has been improved， indicating that the control strategies are effective.