Abstract: In practical engineering, power electronic transformers have gradually become an indispensable and important part of many fields such as new energy grids. The optimization design of high-frequency transformer, as a key device, has become increasingly important. Firstly, the improved Steinmetz empirical formula (IGSE) was used to calculate the core loss of a high-frequency transformer, and the Dowell method was used to calculate the AC winding coefficient of copper foil at high frequency. Secondly, the influences of winding cross transposition on the leakage magnetic field and the conductor current density inside the iron core window were analyzed, the leakage inductance formula based on the energy method was derived, and the six-node thermal network model was established by comprehensively considering the influence of the size of the radiator on the temperature rise. Finally, a high-frequency transformer optimization design method based on free parameter scanning method was proposed, and the Pareto front of the optimization design scheme was obtained. In order to verify the rationality of the design process, a prototype of a 15 kW/5 kHz, efficiency of 99% and power density of 16.79 kW/L, high-power density nanocrystalline high-frequency transformer was designed and manufactured, an experimental platform was built to test the leakage inductance, core loss, and winding loss of the prototype, and it was found that the deviations from the design values were 8.76%, 4.02%, and 2.76%, respectively. The correctness of the optimization design method was verified, which provided a theoretical and experimental bases for the research of high-efficiency, high-power density and large-capacity high-frequency transformers.