In this paper, a modular multilevel DC transformer (AM-MMDC) structure based on multiplexing arm is proposed, which can reduce the capacitance consumption by 25% and improve the utilization rate of submodules compared with the traditional MMDC. AM-MMDC adopts the quasi-two-level modulation, which can realize the functions of voltage matching and power regulation in the future DC distribution network. To solve the dynamic voltage pressure equalization problem of the arm-transfer switch composed of series IGBTs, a specific soft switching strategy is designed to improve the reliability of the arm-transfer switch and the operating frequency of AM-MMDC. Finally, the simulation results prove the correctness and effectiveness of the proposed AM-MMDC structure and its control strategy.