The modular multilevel converter(MMC) based dc-dc converter has been proposed for multi-terminal dc grids. A three-terminal configuration interconnecting three different voltage levels is demonstrated. The operation principle of dc voltage transformation and current regulation is presented. The converter is able to step-up high voltage dc into different dc levels with multiple throughputs and the step-up ratio is derived. Meanwhile, the converter can be connected between several different dc terminals as a power flow controller(PFC) and the current regulation principle is demonstrated. The sub-module capacitor voltages of the chain-link are inherently balanced. The proposed topology is simulated under different circumstances to verify the capability of the multi-terminal step-up dc-dc conversion and the power flow control between multiple dc terminals with different voltage levels.