Abstract: Hard spots in polycrystalline silicon ingots have been studied through experiment and numerical simulation. Morphology and composition of hard spots in polycrystalline silicon ingots were analyzed,and solution was given. The research results shown that the hard spots in the middle of the silicon ingot are generally small in size and mainly consist of SiC;the hard spots in the top of the silicon ingot are large in size and consist of SiC and Si₃N₄,and a small amount of oxygen. Further study shown that the thermal structure of polysilicon directionally solidification ingot furnace has a significant effect on the quality of the silicon ingot. An optimized thermal structure can optimize the solid-liquid interface and reduce the formation of the hard spots,thus improving the quality of the ingots.