A method for manufacturing a polycrystalline silicon ingot includes steps of: A) putting a crucible loaded with a silicon material and having a base and a wall around the base in a thermal field, melting the silicon material in the crucible in the thermal field to form a molten silicon; B) after the step (A), controlling the thermal field to provide heat to the molten silicon from above the crucible and to solidify a portion of the molten silicon contacting a base and at least a portion of the wall proximate to the base of the container to form a solid silicon crystalline isolation layer which surrounds a bottom of the rest of the molten silicon; and C) after the step (B), controlling the thermal field to continuously provide heat to the rest of the molten silicon from above the crucible 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. Therefore in the process for manufacturing the polycrystalline silicon, the solid silicon ingot is pre-solidified and formed, and the final oxygen content in the polycrystalline silicon can be reduced.