A underground granary comprises a roof, a wall and a base; the vertical section of the underground granary is a inverted trapezoid; and reinforcement columns are disposed in the wall. By adopting the inverted trapezoid section, the underground granary can greatly reduce construction difficulty, increase construction speed, and reduce engineering cost; By arranging the reinforcement columns in the wall, the lateral rigidity of the wall is greatly improved, thereby the grain storage depth of the underground granary is increased.