High temperature is essential for the growth of high-quality crystals, and the temperature gradients impact the crystal growth rate, quality, size and so on especially in PVT method. In this study, the size and shape of the cooling hole are optimized for better temperature distribution and lower thermal stress. The thermal field and thermal stress have been simulated by CrysMAS. The results show that the modified cooling hole is superior to the cylindrical cooling hole in the following parts, first, it decreases the radius temperature gradient in crystal; second, it makes the temperature in the furnace more uniform without decreasing the axial temperature gradient; third, the crystal with low thermal stress can be produced.