The present invention discloses a thermal field structure for growing a silicon carbide single crystal, and belongs to the field of preparation of a silicon carbide single crystal. According to the present invention, the thermal field distribution of a PVT method is improved, a conventional method of creating an axial temperature gradient through heat dissipation of upper thermal insulation holesis changed into a method of using different thicknesses of crucibles and different thicknesses of thermal insulation structures to create an axial temperature gradient and at the same time changing the thermal insulation structure on the upper side of the crucibles, so that a thermal field structure with uniform radial temperature distribution is produced, and in particular, the radial distribution of the internal thermal field of a large-size crucible is uniform. Since electroactive impurity elements grow into the crystal with the temperature gradient, the thermal field structure with uniformradial temperature distribution guides the electroactive impurity elements to be evenly distributed in the radial direction, thereby preparing a large-sized high-purity semi-insulating silicon carbide single crystal and single crystal substrate with uniform radial resistivity and low stress.