Abstract: The magnetically coupled three-phase dual- active-bridge (DAB3) DC/DC converter is highly suitable for high-power applications. Although the converter has many advantages, the structure, working mode and electromagnetic effect of its key magnetic component—high-power medium- frequency three-phase transformer (MFT3) is sophisticated, and the capacity, frequency, loss, temperature rise and leakage inductance restrict each other, forming a complex and systematic design problem. This paper focuses on the overall design method and performance of high-power MFT3. Based on the analysis of steady-state voltage and current waveforms of DAB3 converter, the expression of harmonic current of MFT3 is derived by using fundamental wave analysis, and the analytical calculation method of copper loss is proposed. According to the piecewise linear flux density waveform excited by six-step voltage wave, the improved IGSE formula is proposed to calculate the core loss. For MFT3 with three-phase five column core topology, a lumped parameter thermal network model with 14 temperature nodes is established to obtain the maximum temperature rise. The influence of winding arrangement on leakage inductance is analyzed, and the analytical expression of leakage inductance is presented. On this basis, the design method of high-power MFT3 is proposed based on the free parameter scanning method. A 5kHz/15kW MFT3 model with the nanocrystalline core material is made, and the leakage inductance, core loss, copper loss and temperature rise are extracted by finite element method (FEM) and experiment. The effectiveness of the proposed method is verified by comparing the design results with the FEM simulation and experimental results.