The invention relates to a method for purifying crude boron powder. The method comprises the following steps: using the crude boron powder produced by magnesium reduction process as raw materials, mixing the boron trioxide powder into the crude boron powder, mixing, preparing block and purifying the mixture under certain temperature in the horizontal atmosphere furnace; dissolving and rinsing the soluble substances in the left material by hydrochloric acid after cooling, filtrating and drying up to obtain the boron powder with purity of 95-97 percent.. The technology of the invention is reliable, easy to operate and simple in process. The added value of the boron powder is enhanced after purification and the purity of the product can be stabilized at above 95 percent.