The invention belongs to the technical field of boron powder preparation. In order to obtain superfine amorphous boron powder with high purity and low particle size, the invention discloses a preparation method of the superfine amorphous boron powder. The preparation method of the superfine amorphous boron powder specifically comprises the following steps: S1, weighing reaction raw materials, wherein the reaction raw materials comprise the following components in percentage by mass: 40-48% of Mg powder, 35-42% of B2O3 powder and 10-24% of diluent powder, S2, mixing the reaction raw materials,and pressurizing the mixed reaction raw materials to obtain a cake-shaped green body, S3, carrying out a magnesiothermic reaction to prepare a block containing boron powder, and S4, by an acid pickling-water washing immersion cleaning process, carrying out impurity removal on the obtained blocky matter containing the boron powder, so that the high-purity and low-particle-size ultrafine amorphous boron powder is obtained. By adopting the method disclosed by the invention, the ultrafine amorphous boron powder with high purity and low particle size can be obtained, the manufacturing cost can be reduced, and the production efficiency can be improved.