The invention provides a prefabrication assembly type underground granary. The flat shape of the underground granary is rectangular, and the underground granary comprises a granary bottom, multiple prefabricated granary wall plates, a granary top, multifunctional pillars and enhancing pillars, wherein each granary wall plate is a rectangular prefabricated plate, is provided with a connection device on the periphery, and can be connected with one adjacent granary wallboard, one corresponding multifunctional pillar or one corresponding enhancing pillar by means of the connection device; each multifunctional pillar is used for positioning a granary wall, and is a main stress component for bearing soil pressure and grain pressure and resisting buoyance, and the depth of each multifunctional pillar anchored into soil is greater than the depth of a granary bottom plate; the underground granary is characterized in that each multifunctional pillar is divided into a plurality of sections from top to bottom, the inner side of each first section on an uppermost layer is provided with a first-layer granary wall plate, one enhancing pillar is firstly arranged on the inner side of the corresponding second section, the inner side faces of the enhancing pillars are enabled to be flush with the inner side face of the first-layer granary wall plate, then a second-layer granary wall plate is arranged on the inner side face of the corresponding enhancing pillar, and the first-layer granary wall plate and the second-layer granary wall plate are partially overlapped; by using an arrangement mode similar to that above, the second-layer granary wall plate and a third-layer granary wall plate are partially overlapped, and overlapping is performed until a bottommost-layer granary wall plate is overlapped, thus forming the ladder-like assembly type rectangular underground granary.