With the development of traction supply system, the topology of the traditional phase-shift two-level converter can not be suitable for power electronic transformer in high-voltage, high-power application due to its weak voltage stress capacity. Therefore, a proposed topology named bi-directional LLC resonant converter with three-level half bridge is introduced. Compared with previous topologies, it has the advantages of allowing higher input voltage with its higher voltage stress capacity and reducing the switching device stress demand. In addition, it can deliver the power with higher efficiency and density. In order to know more about that proposed topology, the paper also discusses the working characteristics by the analysis method of fundamental frequency and also introduces in detail its working modes under falling resonant and varied modulation frequency, so that the expressions in time domain can be got. Meanwhile, a technique optimizing topology parameters is presented by the circuit principle in dead time. Finally the theoretical analysis and the designed parameters are verified by the simulation and experiment results.