The invention discloses an air purifying device used for a flour processing plant. The air purifying device comprises a shell, a filtering device, an air extracting device, a fine mist purifying device and a sterilizing and disinfecting device, wherein the filtering device, the air extracting device, the fine mist purifying device and the sterilizing and disinfecting device are sequentially installed in the shell. The filtering device comprises a rough filtering layer and a fine filtering layer. A limiting protruding groove for fixing the rough filtering layer and the fine filtering layer is formed in the inner surface of the shell. A sliding groove is formed in the position, at the positions of the rough filtering layer and the fine filtering layer, of the shell. The sliding groove is further provided with a sliding block. The end, penetrating a sliding plate, of the sliding block is fixedly connected with a cleaning brush. Limiting fixing grooves are formed in the two ends of the sliding groove. First fixing threaded holes are formed in the sliding plate and the shell, and the sliding plate and the shell are fixedly installed together through a first sealing ring. A first impurity outlet is formed in the bottom of the shell, and a first closed round piece is hinged to the first impurity outlet. A second sealing ring is arranged on the edge of the first closed round piece, anda sealing protruding groove is formed in the first impurity outlet. The first closed round piece is further connected with a protruding edge.