HVDC transmission is the main transmission form for grid connection and consumption of new energy. Because of the fluctuation of new energy output, the DC voltage fluctuation of an HVDC transmission system will affect its safe and stable operation. To effectively suppress this DC voltage fluctuation, a DC voltage auxiliary control strategy using a variable-speed pumped storage unit is proposed. First, the simulation models of the variable-speed pumped storage unit, four terminal HVDC power grid, wind farm and photovoltaic power station are established. Secondly, the DC voltage deviation multiplied by the corresponding coefficient is used as the micro increment of the active power reference value of the variable-speed pumped storage unit. The influence of the steady-state component of the DC voltage on the DC voltage auxiliary control is filtered through the low-pass filter. An active power control strategy of the variable-speed pumped storage unit based on the DC voltage auxiliary control is proposed. Finally, taking the electric and power generation conditions of a variable-speed pumped storage unit as an example, the ability of the variable-speed pumped storage unit to suppress DC voltage fluctuation is simulated and compared with the traditional direct power control strategy. The simulation results show that the DC voltage fluctuation frequency caused by the output fluctuation of new energy is concentrated below 10 Hz, and the variable speed pumped storage unit control strategy proposed in this paper can effectively suppress the DC voltage fluctuation of an HVDC system. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. NY71-18-013).