When the voltage of the soft open point (SOP) port is unbalanced, it will cause the voltage fluctuation on DC side, which will greatly affect the stability of the entire system. According to the mathematical model of SOP, the cause of the fluctuation is analyzed, and a DC-side voltage suppression strategy based on second-order sliding mode control is proposed in this paper. The precise tracking of reference voltage and current is carried out by introducing a capacitor compensator device based on a nonlinear disturbance observer. Considering a variety of port voltage imbalance conditions, an SOP simulation model is built in MATLAB/Simulink. The simulation results show that the proposed method has a significant effect on the suppression of DC-side voltage fluctuations.