Solid-state transformers (SSTs) can be applied in the ac/dc hybrid distribution network. The reliability and power density of a solid-state transformer is significantly influenced by the insulation design of the isolated dc-dc converter and, in particular, its medium-voltage medium-frequency transformer. This article proposes an insulation design method for the medium-voltage medium-frequency transformer. A novel insulation structure is adopted to reduce the volume and improve the heat dissipation performance. Optimization of insulation distance parameters is performed based on finite-element method (FEM) simulations. The proposed method is applied to a 60-kVA/10-kHz transformer, and the simulation results verify the improved performances with the proposed insulation design method. A prototype is developed according to the proposed scheme, and the experimental results of a dc-dc converter and the partial discharge and applied voltage tests are presented.