This paper proposes a novel medium-frequency coaxial transformer design method to solve the high-voltage insulation problem between the high-voltage side and low-voltage side windings in DC/DC converters. Considering the actual situation in all-DC offshore wind farms, a medium-frequency coaxial transformer for modular isolated DC/DC converter (MIDC) is designed. At first, the coaxial transformer structure is analyzed and parameter optimization is made in detail. Then, the optimal solution for the coaxial transformer is obtained. Moreover, simulation of the transformer temperature field is done with Ansys software. Finally, a rated 250 kVA medium-frequency coaxial transformer is set as a design example. The result shows that the optimal transformer volume is 0.0223 m³ with a weight of 47.11 kg, and the maximum temperature is 98.5 °C under full-load working condition, with both the magnetizing inductance and the leakage inductance values meeting the design requirements. The simulation proves the effectiveness and feasibility of the design method proposed in this paper.