The invention discloses a high-thermal-conductivity boron nitride powder filler and a preparation method thereof. The high-thermal-conductivity boron nitride powder filler is prepared from sodium hydroxide, boron nitride, an epoxy silane coupling agent, absolute ethyl alcohol and deionized water. The mass percentage of each component is as follows: 15 to 25 parts of sodium hydroxide, 40 to 60 parts of boron nitride, 20 to 30 parts of epoxy silane coupling agent, 10 to 20 parts of absolute ethyl alcohol and 5 to 10 parts of deionized water. The preparation method is safe and reliable, the microsphere boron nitride easily forms a heat conduction network, the particle size of the boron nitride is optimized, the heat conduction effect of the filler is improved, the wet modification process conditions of the epoxy silane coupling agent on the boron nitride powder are optimized, so that the boron nitride powder has good compatibility with organic matters, and the viscosity, the oiliness, the water content and the like meet the technical index requirements; and the process is simple and rigorous, can replace other expensive high-thermal-conductivity boron nitride powder filler production processes, has the advantages of significant effect, cheap raw materials and low cost, greatly saves the production cost, and is beneficial to processing and production.