A method of preparing cerium boride powder, according to the present invention, includes a first step for generating mixed powder by mixing at least one selected from among cerium chloride (CeCl3) powder and cerium oxide (CeO2) powder, at least one selected from among magnesium hydride (MgH2) powder and magnesium (Mg) powder, and boron oxide (B2O3) powder, a second step for generating composite powder including cerium boride (CexBy) and at least one selected from among magnesium oxide (MgO) and magnesium chloride (MgCl2), by causing reaction in the mixed powder at room temperature based on a ball milling process, and a third step for selectively depositing cerium boride powder by dispersing the composite powder in a solution.