This paper proposes a trapezoidal current working mode of modular multilevel DC-DC converter (MMDC) with bidirectional power flow capability, which is attractive for the interface requirement of future High Voltage Direct Current (HVDC) networks. Through particular modulation methods, it achieves a flexible transmitted power and inherent balance of sub-modular (SM) capacitor voltage. In addition, soft-switching and low operating current, which means low switching losses and conduction losses respectively, are guaranteed by its trapezoidal current waveform. The modulation method and operation principle of this converter are analyzed. An example of 12 kV to 1 kV, 100 kW converter is designed. In addition, its pros and cons are analyzed through comparative study.