In order to reduce the dislocation density of SiC single crystal grown by PVT method, the thermal field was simulated based on finite element analysis. In this paper, VR-PVT SiC software was used to simulate the internal temperature gradient and thermal stress evolution caused by the different crucible structure. The change of crucible lid and sidewall structures has a better influence on radial temperature gradient and axial temperature gradient, respectively. Modified structure is a combination of these structures which improves both axial and radial temperature gradient. It reduces the maximum stress from 1.85 MPa to 1.58 MPa. The maximum shear stress and dislocation density distribution are shifted to the edge of the crystal. The optimization has only changed the external structure, so there is no conflict with the internal design.