The medium-voltage solid-state transformers (SST) have played an important role in medium-voltage systems such as smart micro grids,renewable energy systems,traction systems and so on. Compared with traditional 50/60 Hz transformers,high operating frequency can greatly reduce the volume and weight of magnetic components. In this thesis,the three-level half bridge topology lets the voltages across the semiconductor elements are half of input voltage,therefore,the converter can withstand higher input voltage. The CLLC resonant structure realizes soft switching that reduce the switching loss to higher the converter's efficiency and bidirectional power transfer. In addition,the insulation system is designed to protect the converter and driving circuit from damaged when the circuit breaks down. A prototype circuit is made in this thesis. The high voltage side is 5.2 kV,the low voltage side is 400 V and the rated power is 40 kW. The cast resin transformer is used as isolation transformer in the circuit. Finally,the experimental high-voltage side 2.6 kV and the low-voltage side 200 V are used to verify and support the theoretical formula and the SIMPLIS simulation results.