Epoxy resin (EP) is a common insulate material with excellent insulating property and high adhesion, attracting considerable attention in the insulation structure of solid-state transformers. However, inevitable temperature rise results from the poor thermal conductivity can cause a deterioration of operational reliability of devices when work under high-frequency condition. In this work, epoxy resin was used as the main matrix, while micro-boron nitride (BN) and nano-alumina (Al2O3) were introduced as additives. The micro-nano co-doped epoxy resin with various BN/Al2O3 content were prepared by in-situ polymerization method. The thermal conductivity(σ) and voltage resistance of co-doped EP were focused on and investigated systematically. The best thermal conductivity (1.21 W·m−1·K−1) took place in the samples with the micron-BN content of 27 wt% and nano-Al2O3 content of 3 wt%, improved by 476% compared to the pure EP. Furthermore, micro-nano co-doped composites with a thickness of 0.2 mm can withstand a bipolar square-wave voltage of 10 kV and 20 kHz for up to 230 s, representing an 820% improvement compared to pure EP. All the results demonstrate that there is an obvious effect of micron/nano additives on σ and high-frequency insulation properties of epoxy materials. The significant improvement of relevant performance is crucial for ensuring the stability of solid-state transformers in specific high-frequency applications.