Polycrystalline silicon prepared by the directional solidification method is currently the main photovoltaic raw material. During the preparation process, the thermal field structure and the control of the convection morphology of the silicon melt are extremely important for the growth of high-quality polycrystalline silicon. This article uses professional crystal growth software CGSim to prepare solar-grade polycrystalline silicon with vacuum. The graphite crucible in the induction casting furnace was improved and numerical simulation was carried out. The influence of the thickness of different graphite crucibles on the thermal field, flow field, solid-liquid interface, silicon crystal stress field, and V/G value was analyzed. The results show that when the thickness of the graphite crucible is 20 mm, good convection morphology, flat solid-liquid interface, reasonable V/G value, etc. can be obtained, which are beneficial to save the production cost of polysilicon and improve the quality of polysilicon. It provides an important theoretical basis for process optimization and defect aualysis in production practice.