The utility model relates to an underground granary, which comprises a granary roof, a granary wall and a granary base. The granary wall is a rectangle and a reinforcing post is arranged in the granary wall. Owing to the adoption of the rectangle granary wall, construction difficulty of the underground granary can be reduced greatly, the construction speed can be accelerated, and the construction cost can be reduced; and owing to the reinforcing post arranged in the granary wall, the granary wall has the advantage of better lateral rigidity, and the interior storage space of the underground granary is enlarged at the same time.