NOVELTY - Preparation of polycrystalline silicon ingot, involves: after filling silicon material in crucible, heating, melting the silicon, obtaining silicon melt, adjusting heat field, and nucleation crystallizing the silicon melt on the crucible.    USE - For preparation of polycrystalline silicon ingot (claimed) useful as photovoltaic material for solar battery industry; and in the field of manufacturing semiconductor.    ADVANTAGE - The polycrystalline silicon prepared by this method, has few dislocations. The preparation method is specifically applicable for casting large-size high silicon ingot. The polycrystalline silicon slice is prepared by taking the polycrystalline silicon ingot as raw material. The preparation method can effectively reduce the dislocation and defect density of the polycrystalline silicon ingot head to obtain dislocation at the high quality polycrystalline silicon ingot; is especially suitable for casting large size high silicon ingot; and obtains several crystal starches as raw material of multi-crystal sheet.    DETAILED DESCRIPTION - Preparation of polycrystalline silicon ingot, involves: after filling silicon material in crucible, heating, melting the silicon, obtaining silicon melt, adjusting heat field, and nucleation crystallizing the silicon melt on the crucible; where the process for nucleation crystallizing involves the steps as follows: first crystallizing period: after growing the crystal with target height, adjusting heat field, stopping the first crystallizing period, before not completely melting the crystal grown from the first crystallizing period, adjusting heat field, forming supercooled state, nucleation crystallizing the uncrystallized silicon melt on the crystal formed in the first crystallizing period; and carrying out further nucleation crystallizing process being subsequent crystallizing period, after crystallizing all silicon melt, annealing, cooling and obtaining polycrystalline silicon. The subsequent crystallizing period involves multiple repeated nucleation crystallizing processes. INDEPENDENT CLAIMS are included for the following:    (1) polycrystalline silicon ingot, which is obtained by the method; and    (2) a polycrystalline silicon slice, which is obtained through extracting, slicing and washing the polycrystalline silicon ingot.