With the trend of modern electronic device miniaturization and higher power density, polymer composites with both high thermal conductivity and excellent dielectric properties are highly desirable. In this work, a homogeneous, porous, and insulating Si3N4w@SiO2 framework was prepared using silicon nitride nanowires (Si3N4w) by a polystyrene (PS) template method modified with fumed silica (SiO2). After infiltration, the thermal conductivity of the epoxy/3D-Si3N4w@SiO2 composites reached 1.05 W/m·K at the loading of 7.80 vol% (5.25 times higher than pure EP), and showed good thermal stability. In addition, the dielectric properties of the composites were systematically investigated. The composites show excellent dielectric properties, which can maintain dielectric constant below 5.5 and loss tangent below 0.05 in a wide frequency range (>1 Hz), temperature range(<120 °C) and under high electric field intensity (106 V/m). These results demonstrate the potential of Si3N4w for application in electronic packaging materials and provide a simple preparation strategy.