Large-size, low-defect silicon carbide single crystal is one of the most important fundamental materials for power and radio frequency (RF) devices. The physical vapor transport (PVT) method is the major technique for growing large-size SiC single crystals currently. The core to obtain large-size and high-quality crystals is to find optimal matching conditions for vapor composition, temperature, and pressure at the crystal growth interface so that the vapor can crystallize evenly, at the same time the thermal stress in crystals is sufficiently small. This paper presents the numerical studies of the thermal field design for 8 inch SiC bulk crystal growth system with resistance heating. Specifically, the influences of heater position, heating power and the radiation aperture’s diameter on thermal field were studied, in conjunction with the optimal system structure. Numerical simulation results show that optimizing design parameters such as the shape of heat dissipation hole and the structure of insulation can achieve thermal fields with low horizontal temperature gradient and high axial temperature gradient desirable for the growth of large-size SiC crystal by resistance heating system while the thickness of grown crystal changes and porous raw material is consumed.