Abstract: The calculation of the operating loss of power electronic converter is an important step to optimize its design and reduce the operating cost, to carry out practical application in engineering. A kind of high voltage DC transformer topology which is suitable for offshore wind power DC transmission is introduced briefly and the special S/M modulation principle is analyzed. Then, a complete loss calculation model is established for the proposed composite modular DC converter based on MMC-DAB module, and the reliability of the modulation strategy and the correctness of the loss calculation model are verified by simulation. Finally, the rated operating conditions of the converter are designed, and the parameters of the actual switching devices are used to calculate the operating efficiency of the converter, which lays the groundwork for actual application and efficiency improvement of the proposed composite modular DC converter.