A method for manufacturing a polycrystalline silicon ingot includes steps of: a) melting a silicon material in a container disposed in a thermal field to form a molten silicon; b) controlling the thermal field to provide heat to the molten silicon from above the container and to solidify a portion of the molten silicon contacting a base part and at least a portion of a wall part proximate to the base part of the container to form a solid silicon crystalline isolation layer; and c) controlling the thermal field to continuously provide heat to the rest of the molten silicon from above the container and to solidify the rest of the molten silicon gradually from a bottom to a top of the rest of the molten silicon to form a polycrystalline silicon ingot.