The converter station of the LCC-MMC hybrid UHVDC transmission system with the LCC rectifier and the MMC inverter is formed in series by high and low voltage valve groups. When this system uses two control strategies: “constant current control in LCC converter/constant voltage control in MMC converter” and “constant voltage control in LCC converter/constant eactive power control in MMC converter”, there is a phenomenon that the DC voltage and active power of the high and low valve groups of the LCC rectifier and the MMC inverter are unbalanced. This paper proposes that the unbalanced current between the high and low valve groups and the uncontrollable voltage of valve is the cause of the DC voltage imbalance between the valve groups, and analyzes the mechanism of the unbalance phenomenon. A voltage equalization control strategy using the deviation between the actual voltage of the valve group and the rated voltage is designed, so that the DC voltage between the valve groups can be balanced. Finally, the effectiveness of the proposed voltage equalization control strategy is verified on the PSCAD/EMTDC platform.