Modular multilevel DC transformers can interconnect DC lines of different voltage levels and types, serving as a hub for high-voltage DC collection and transmission. Currently, research on modular DC transformers is still in the stage of circuit topology design and functional verification, and a unified, general controller design method has not been established in terms of control. Therefore, this paper takes the average value model of a modular DC transformer as the research object. It first analyzes the characteristics of the topology structure and the principle of voltage transformation. Then, it derives the common /differential constraint relationships of AC components and the dynamic characteristic equations of internal and external electrical quantities. Finally, aiming to achieve energy balance among the submodules of each bridge arm and reduce the amplitude of internal circulating current, a hierarchical control strategy is proposed to decouple and operate the controllers independently, achieving the goals of power transmission and stable voltage transformation. Lastly, the effectiveness of the proposed control method is verified in MATLAB/SIMULINK.