Abstract: To solve the problem of increase of isolation transformer losses led by a large number of high harmonics existing in the AC side voltage and current of quasi two-level modulation strategy in modular multilevel DC/DC converter (abbr. MMDC), firstly, the relationship between the switching angle of the power device and the total harmonic distortion (abbr. THD) of the primary side line voltage of the transformer was obtained by analysis. Secondly, a THD optimization modulation strategy with variable duty cycle and amplitude was proposed. The THD of the primary side line voltage and current of the transformer were reduced by changing the switch angle to reduce the transformer losses. Finally, the feasibility of the proposed THD-based the optimal modulation control strategy was verified by simulation software PSCAD/EMTDC.