Disclosed is a method for preparing polycrystalline silicon ingot. The preparation method comprises: coating inner wall of the crucible with a layer of silicon nitride, followed by laying a layer of crushed silicon and feeding silicon in the crucible; the crushed silicon is laid in random order, and the layer of crushed silicon forms a supporting structure having numerous holes; melting the silicon to form molten silicon by heating, when solid-liquid interface reach the surface of the layer of crushed silicon or when the layer of crushed silicon melt partially, regulating thermal field to achieve supercooled state to grow crystals; after the crystallization of molten silicon is completely finished, performing annealing and cooling to obtain polycrystalline silicon ingot. By adopting the preparation method, a desirable initial nucleus can be obtained for a polycrystalline silicon ingot, so as to reduce dislocation multiplication during the growth of the polycrystalline silicon ingot.