Winnowing machine with air return pipeline convenient to clean

The winnowing machine comprises a separation chamber, a diffusion chamber, a draught fan, the air return pipeline, a valve device, a first control mechanism and a second control mechanism, the diffusion chamber is located on one side of the separation chamber and communicated with the separation chamber, a plurality of air suction boxes are arranged above the diffusion chamber, and the air suction boxes are communicated with the interior of the diffusion chamber; the fan is provided with a first air inlet and a second air inlet, an air outlet of the fan is communicated with a first air outlet pipe and a second air outlet pipe, and the second air outlet pipe is communicated with the separation chamber; the valve device can control opening and closing of the second air inlet; the air return pipeline communicates with all the air suction boxes, the end, away from the air suction boxes, of the air return pipeline communicates with a first air inlet of the fan, and the end, away from the air suction boxes, of the