In the LCC-MMC hybrid multi-terminal UHVDC system, the feed end adopts conventional DC, and the receiver end adopts flexible DC. The voltage imbalance will appear between two series valve groups in the constant active power MMC station during valve deblock. The mechanism of this imbalance phenomenon is analyzed, and then, a voltage balancing strategy based on correcting the voltage deviation between the valves is proposed in this paper. This strategy adjusts the deviation value of the high and low valves by introducing a function whose decreasing rate is the same as the boost rate of the voltage, and converts it into the active power deviation value. Moreover, the value is then added to the initial active reference value as the reference value of the active outer ring, and finally, the on-line steady input of the valve group is realized and the transition to steady state is achieved. Based on the PSCAD/EMTDC simulation platform, a model of the three-terminal hybrid UHVDC transmission system is built, and the effectiveness of the proposed strategy is verified by comparing the impact of three different voltage balancing strategies on the dynamic characteristics of the system.