The invention relates to an automatic seed and seedling dropping device and a control method thereof. The device comprises a fixed plate, a sliding plate, limiting guide rails and a control system, wherein the fixed plate, the sliding plate and the limiting guide rails are mounted up and down in sequence in a stacking manner; rectangular through holes are formed in the fixed plate; the sliding plate is mounted below the fixed plate in a parallel manner; two grooves are formed in two sides of the bottom surface of the sliding plate along the length direction; a section of rack is arranged at the left end of one side of the sliding plate; the limiting guide rails consist of supporting plates and pulleys; the pulleys are mounted between the supporting plates and the sliding plate; the control system comprises a single-chip microcomputer, a stepping motor, a motor gear and an encoder; the stepping motor operates to drive the sliding plate to transversely slide left and right along the limiting guide rails through meshing transmission between the motor gear and the rack. The automatic seed and seedling dropping device can automatically control dropping of garlic seeds or pot seedlings according to a displacement of a machine, so that the planting distance of the garlic seeds or the pot seedlings can be precisely controlled, and reseeding and missing seeding of crops can be avoided.