The series HVDC tap based on modular multilevel converter (MMC) has been proposed to provide power supply for rural places. However, most of current capacitor voltage balancing control strategies are not suitable for series MMC tap. In this paper, the principle of power transfer between phases, arms and sub-modules (SMs) in a series MMC are analyzed. A hierarchical capacitor voltage balancing control strategy is then proposed which can automatically balance the voltage of each SM capacitor. Particularly, the control layers are decoupled from each other to improve the stability of the system. Finally, correctness of the proposed control strategy is proved by simulation results.