A prefabrication assembly type underground granary comprises a granary bottom, multiple prefabricated granary wall plates, a granary top, multifunctional pillars and multiple layers of enhancing pillars, wherein each granary wall plate is of an integral structure formed by combining a plastic plate on the outer side and a concrete slab on the inner side, each granary wall plate is provided with connection devices on the periphery, and the granary wall plates can be connected with adjacent granary wallboards, the multifunctional pillars and the multiple layers of enhancing pillars by means of the connection devices; the multifunctional pillars and the enhancing pillars are wrapped by plastic plates, can be used for positioning the granary wall plates, and are main stress components for bearing soil pressure and grain pressure and resisting buoyance, and the depth of each multifunctional pillar and each enhancing pillar anchored into soil is greater than the depth of the granary bottom; 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, the corresponding enhancing pillars are 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 underground granary.