This paper proposes a trapezoidal voltage mode for bidirectional modular multilevel DC-DC converter (MMDC), which combines the benefits of modular multilevel converter (MMC), resonant modular multilevel converter (RMMC) and dual active bridge (DAB). It is feasible for the connection between Medium Voltage Direct Current (MVDC) transmission lines and Low Voltage Direct Current (LVDC) transmission lines. With proper modulation, its current is a trapezoidal waveform, which guarantees low switching losses and conduction losses. Due to the removing of shunt capacitor, the fault produced by short circuit can be handled. With proper modulation, inherent balancing of SM capacitor is achieved. In addition, the voltage waveform of transformer is trapezoidal, so the saturation issue is moderated. To illustrate its pros and cons, this converter is compared with dual active bridge (DAB) converter through a case study (30 kV to 1 kV, 1 MW).