The invention discloses a preparation method of a nuclear grade boron carbide core block for a high-temperature gas cooling pile control rod. Nuclear grade boron carbide powder is used as raw materials; the large-dimension nuclear grade boron carbide core block for the high-temperature gas cooling pile control rod is prepared through nine steps including material mixing, drying, pelletization, steel mold forming, cold isostatic pressing, green blank processing, degreasing, sintering and machining; the large-dimension nuclear grade boron carbide core block can be prepared in a mass and net dimension manner; a pressure-free sintering process is used for replacing a hot pressing process, so that the problems of high production cost and low efficiency of the large-dimension nuclear grade boron carbide core block are solved. The method can be directly used for producing the boron carbide core block for the high-temperature gas cooling pile control rod.