An auto-feeding device includes a container provided on one side with a hollow sleeve; a bottom cover operably connected to an open bottom of the container; a fixing seat having an upright rod upwardly rotatably extended into the hollow sleeve and a stop rod normally located below the bottom cover; and a swaying arm upwardly connected to the fixing seat opposite to the stop rod. When the auto-feeding device is mounted on a bowl of a food processor filled with water, and blades in the bowl are rotated at a preset time to produce swirling water, the swaying arm is pushed by swirls to move the stop rod away from the bottom cover, so that the bottom cover turns downward to open the bottom of the container, allowing foodstuffs in the container to automatically fall into the bowl and be directly heated and ground into different types of food juice.