The invention belongs to the technical field of electrolytic metallurgy, and particularly relates to a method for preparing high-purity nano hafnium boride from molten salt through electrolysis. The preparation method comprises the steps of taking hafnium oxide and boron as raw materials, and mixing and compressing hafnium oxide powder and the boron into porous blocks; compounding the porous blocks and a cathode collector to be regarded as a cathode, and taking graphite as an anode; and electrolyzing the molten salt by taking one or more of CaCl2 and NaCl as molten salt electrolytes to directly generate the hafnium boride, and the prepared hafnium boride product is high-purity nano powder. The preparation method provided by the invention has the advantages of lower reaction temperature, easy operation, and low price of the raw materials, and is environmentally friendly and beneficial to industrial continuous production.