Abstract: Epoxy composites with high thermal conductivity have broad application prospects in the fields of electronic packaging and electrical equipment, but the improvement of thermal conductivity of composites is often accompanied by the deterioration of dielectric properties. Using silicon nitride whiskers (Si₃N_4w) and fumed silica (SiO₂) as raw materials, this paper prepares a solid, porous and insulating Si₃N_4w skeleton with improved polystyrene (PS) template method, and the epoxy composites are obtained after vacuum infiltration. The results show that the thermal conductivity of the composites containing Si₃N_4w skeleton is 1.05 W/(m·K) with a low filler content of 7.80%, which is 525% higher than that of pure epoxy. At the same time, the dielectric properties of the composites are systematically studied under different electric field intensities (10³~10⁶ V/m) and temperature ranges. Owing to the insulating skeleton, the composites keep their dielectric constant and loss tangent below 5.5 and 0.05 under 120℃, showing excellent dielectric properties. It is found that the dielectric constant and loss tangent of the composites increase slightly with the increase of electric field intensity. Meanwhile, both of them increase with the increase of temperature and jump near the glass transition temperature, subsequently losing their insulating property.