The invention provides a polycrystalline silicon ingot casting device and a preparing method of a silicon ingot. The device comprises a quartz crucible and a plurality of combined protection plates arranged on the periphery of the quartz crucible. Each combined protection plate comprises a graphitic layer and a metal isolation layer arranged on the graphitic layer, wherein the metal isolation layer is in contact with the quartz crucible. According to the polycrystalline silicon ingot casting device, the metal isolation layers are arranged on the graphitic layers and can isolate the dry reaction between the quartz crucible and graphite to generate carbon compounds, the carbon content in the silicon ingot is reduced, and thus the effective utilization rate of the silicon ingot is increased; the grade-A rate of polycrystalline silicon wafers obtained by slicing the silicon ingot is increased, and the line broken rate is reduced. It is shown through experimental results that by means of the polycrystalline silicon ingot casting device, the carbon content in silicon wafers is reduced by 1.5-3 ppm, the effective utilization rate of the silicon ingot is increased by 1.2-3.5%, meanwhile, the line broken rate of the silicon wafers is reduced by 0.5-2.5%, and the grade-A rate of the silicon wafers is increased by 0.5-1.5%.