The invention discloses a boron carbide-aluminum alloy composite material board and a preparation method thereof. The preparation method comprises the following steps: (1) preparation of a billet: preparing a boron carbide-aluminum alloy composite material billet; (2) heat treatment: conducting heat treatment on the billet prepared in the step (1) to reach the temperature that the boron carbide-aluminum alloy composite material billet is deformable basically but not molten; (3) rolling: rolling the billet after the heat treatment in the step (2) at a pass reduction of 5-20%; (4) repetition of step (2) and step (3) to obtain the boron carbide-aluminum alloy composite material board of which the thickness is 0.5-8 mm. The board prepared by the method is uniform in formation and compact in structure, has superior mechanical property and decay resistance, cannot generate hydrogen releasing bubbles during a neutron absorption process leading to bulge of the board, and is suitable for safe transportation and long-termed storage of spent fuel.