The invention provides a grain-size sand bin unloader. The grain-size sand bin unloader comprises a main machine body, wherein the top of the main machine body is connected with a top rotary motor through an L-shaped upper supporting frame; the bottom of the top rotary motor is connected with a bowl-shaped suction nozzle through a vertically-arranged main supporting strut; four semicircular openings are uniformly formed in the outer edge of the bottom of the bowl-shaped suction nozzle; an annular guide rail is arranged on the side wall of the bowl-shaped suction nozzle; a sliding block which slides along the annular guide rail is arranged on the annular guide rail; the sliding block is connected with a sealing cloth strip arranged at the outer part of the annular guide rail in a sleeving manner; a rubber hose extending into the bowl-shaped suction nozzle is connected to the sliding block; the other end of the rubber hose communicates with a suction pump arranged in the main machine body; and the suction pump communicates with a material storage chamber.