The invention relates to a method for preparing high-purity boron carbide powder by a solid-phase method. Particularly, the preparation method comprises: mixing a high molecule carbon source and a boron source according to a mass ratio of (1 to 10) to 1; carrying out first heating processing; carrying out second heating processing; carrying out first cooling processing; carrying out third heating processing; carrying out fourth heating processing; carrying out second cooling processing; carrying out fifth heating processing; carrying out third cooling processing. The method disclosed by the invention solves the problems of complex process, high energy consumption and the like in the production process of the boron carbide powder. Moreover, the method disclosed by the invention avoids the problems of low product purity, difficulty in grinding processing of a product, low product yield and the like in the conventional boron carbide powder preparation process, and meanwhile, avoids the problems that when a polymer precursor method is adopted to prepare boron carbide, a precursor needs to be prepared firstly, a treatment process is complex and the like. Therefore, the preparation difficulty of the boron carbide product and the like is greatly reduced, and product quality is improved.