This paper discusses the analysis and design of a multi-port DC-DC converter using Gallium Nitride transistors for a 350V bipolar DC grid application, which could be used as the first stage to interconnect a 350V bipolar DC grid and two electric vehicle batteries. The multi-port DC-DC converter is designed with a three-level neutral-point-clamped triple-activebridge topology. The converter's parameters are selected on the basis of its performance characteristic and system specifications. Moreover, a simulation model is built to analyze the design. In the end, a prototype converter is built and the preliminary experimental results of it are shown and discussed.