Abstract: The existing topology of power electronic transformer (PET) demands a considerable number of high-frequency transformers and a large volume. To address this issue, we adopt a high-power-density PET structure, which merely requires one high-frequency transformer to achieve a high degree of PET integration. To guarantee the stable operation of the proposed topology, we analyze the mechanism of capacitor voltage fluctuation of the submodules of the modularized multilevel converter and propose a method to reduce the capacitor voltage fluctuation of the submodules under the ultra-high modulation ratio. Meanwhile, aiming at the issue that the correlation between the internal phase shift angle and the exchanged power between the bridge arms is influenced by the power phase shift angle in the modularized multilevel DC converter, we put forward a parameter design method of the bridge arm inductance, which can lower the complexity of the control. Moreover, a simulation model is constructed in MATLAB/Simulink to verify that the proposed method can be adopted to reduce capacitor voltage fluctuation and achieve voltage equalization under all working conditions. The results verify the effectiveness of the proposed method.