A tank of the spreading device is rotated by a driving device. The tank contains a number of side boards, each of which is composed of a static plate and a door plate hinged to the static plate. The hinge joining the door and static plates has an elastic element preventing the door plate from being opened outward. When the tank is rotated at a high speed, a centrifugal force on the feedstuff inside the tank is strong enough to push the door plates open and the feedstuff is thereby automatically cast out. By varying the speed of the tank's rotation, the feedstuff could be cast for various distances, thereby achieving efficient and effective distribution of the feedstuff.