Abstract: The poor thermal conductivity and heat resistance of liquid crystalline epoxy resin make it difficult to dissipate heat during the operation of electric and electronic devices, and the rising temperature leads to the deterioration of epoxy resin insulation, which greatly impairs the reliability and life of electric and electronic devices. The electrospinning method and process for preparing high orientation liquid crystalline epoxy fiber thin films were developed in this paper. Nano boron nitride (BNNSs) was filled into the fiber films by vacuum filtration method, and thermal-conductive thin films which were filled with the oriented nano boron nitride/liquid crystalline epoxy fiber composite were further prepared. The effects of liquid crystalline epoxy fiber diameter and nano boron nitride filling concentration on the thermal properties and the AC breakdown strength of BNNSs/liquid crystal epoxy resin composite films were studied. The results show that the in-plane thermal conductivity of liquid crystalline epoxy resin fiber film increases with the decrease of fiber diameter. When the fiber diameter decreases to 280 nm, the thermal conductivity is 0.699 W/(m∙K). When the BNNSs thermal conductive filler is filled, the in-plane thermal conductivity of BNNSs/liquid crystalline epoxy resin thermal conductive composite film increases sharply with the increase of filler concentration. When the filler content is 15%, the in-plane thermal conductivity can reach 5.88 W/(m∙K), which is 779% higher than that of diameter. At the same time, it is found that the AC breakdown strength of the liquid crystalline epoxy resin fiber film is high when the diameter is small, and the breakdown strength of the 280 nm fiber film is 26.55 kV/mm. The addition of BNNSs thermal conductive filler can reduce the thermal breakdown of the film composite. When the filler content is 10%, the power frequency breakdown strength increases most significantly, which increases by nearly 11%, reaching 29.47 kV/mm. Therefore, it is demonstrated that BNNSs-filled liquid crystal epoxy resin composite film materials can significantly enhance thermal conductivity while maintaining excellent electrical insulation properties.