The invention relates to the technical field of ceramic material preparation, and relates to a preparation method of high-thermal-conductivity aluminum nitride ceramic. According to the invention, aluminum nitride powder is adopted as a base material, and rare earth metal fluoride EuF3, LaF3, SmF3 or a mixture thereof is adopted as a sintering aid. An addition amount of the sintering aid rare earth fluoride accounts for 2-8% of the aluminum nitride powder by mass. Through the steps of wet milling mixing, drying, forming, degreasing and sintering, the aluminum nitride ceramic is formed. The aluminum nitride ceramic has thermal conductivity higher than 200W/m.K, flexure strength greater than 320MPa and a fine grain size. The method provided by the invention has the advantages of simple process, good product performance, low production cost, and the like.