The present invention relates to a high-purity ultrafine zirconium diboride powder material and its preparation method. It is characterized by that the described powder material contains zirconium oxychloride, boron carbide powder and active carbon powder, their mole ratio is ZrOcl2:B4C:C=1:0.4-0.8:1.4-1.8. Its preparation method includes the following steps: firstly, mixing boron carbide powder, active carbon powder and water according to mixing ratio, regulating pH value to obtain mixed suspension; dissolving zirconium oxychloride in deionized water to obtain the zirconium oxychloride solution; mixing the above-mentioned suspension and zirconium oxychloride solution, adding ammonia water, making the zirconium oxychloride be fully hydrolyzed and precipitated; making suspension undergo the processes of solid-liquor separation, waster-washing, removing NH4+ and Cl-, drying and sieving; then placing the powder material into a vacuum furnace to make reaction synthesis, its synthesis temperature is 1500-1600 deg.C, heat-insulating for 0.5-4 h; grinding powder and sieving so as to obtain the invented powder material.