The professional modeling software package CrysVUn was employed to study the process of a large sapphire single crystal growth using Kyropoulos method. The influence of gas pressure on thermal field, solid-liquid interface shape, gas velocity field and von Mises stress were studied for the first time. It is found that the root of the seed melt when gas pressure equals to one atmosphere or more than one atmosphere, especially during the seeding period, this result is consistent with the experimental observation, and this paper presents three ways to solve this problem. The temperature gradient and stress decreases significantly as the gas pressure increases. The convexity of the solid-liquid interface slightly increases when the gas pressure increases. Numerical analysis was used to optimize the hot zone design. The professional modeling software package CrysVUn was employed to study the process of a large sapphire single crystal growth using Kyropoulos method. The influence of gas pressure on thermal field, solid-liquid interface shape, gas velocity field and von Mises stress were studied for the first time is the root of the seed melt when gas pressure equals to one atmosphere or more than one atmosphere, especially during the seeding period, this result is consistent with the experimental observation, and this paper presents three problems to solve this problem . The temperature gradient and stress decreases significantly the gas pressure increases. The convexity of the solid-liquid interface slightly increases when the gas pressure increases. Numerical analysis was used to optimize the hot zone design.