The invention discloses a method for preparing pure boron. The method comprises the following steps of; mixing 39.0 to 40.7 mass percent of Al, 36.0 to 44.3 mass percent of B2O3, and 15.0 to 25.0 mass percent of KCLO3, wherein the raw materials are powder; placing the power used as raw materials in a ball-milling device and mixing for 8 to 16 hours, placing the reaction materials in a mould and compacting under the pressure of 10 to 60 MPa, and placing an igniting agent on the surface of the reaction material; then reacting in a high pressure autoclave, blowing the high pressure autoclave by argon at room temperature to remove air from the high pressure autoclave, after the temperature of the container rises to 180 DEG C, discharging air again, then introducing argon with pressure of 2 MPa, and raising the temperature of the container continuously; when the temperature in the container reaches about 260 DEG C, allowing the igniting agent to react to release a large amount of heat so as to initiate the reaction between reaction materials; and after the reaction is finished in several seconds, performing furnace cooling on the generated product under the protection of the argon to room temperature to obtain materials containing boron, wherein the boron content of the product is not lower than 91.7 percent.