The invention relates to the technical field of garbage incineration power generation, in particular to a compressing and downsizing device for cured materials of fly ash. The compressing and downsizing device comprises a compression box with two open ends. A slot is formed in one end of the compression box, a discharge door is inserted in the slot, and a compression block is arranged in the compression box. A horizontal first hydraulic cylinder positioned in the compression box is arranged on one side, back on to the discharge door, of the compression block. The hydraulic rod end of the first hydraulic cylinder is connected to the middle of the compression block. A support guide wheel is connected to a cylinder barrel of the first hydraulic cylinder, and the first hydraulic cylinder is supported at the bottom of the compression box through the support guide wheel. A feed inlet is formed in the middle of the upper end face of the compression box, a feed hopper is connected to the feed inlet, and a sealing door is arranged at the outlet end of the feed hopper. One end of the sealing door is hinged to the compression box, and a bolt is arranged at the other end of the sealing door. A slot matched with the bolt is formed in the inner wall of the feed hopper. The compressing and downsizing device has the advantages that the size of the cured materials of the fly ash is reduced and landfilling ground of a landfilling factory is saved.