In this paper, type-Ib diamond single crystals, using two different sizes of synthesis cavities of ?15 mm and ?30 mm, were synthesized under 5. 7 GPa, 1 560 ~ 1 600 K, and the temperature field distributions of the two sizes of synthesis cavities were studied using finite element method (FEM). Firstly, the temperature field distributions of two different sizes of synthesis cavities were simulated using finite element method. The average radial temperature gradient and average axial temperature gradient of the ?15 mm synthesis cavity are 0. 573 and 5. 700 K/ mm, respectively. The average radial temperature gradient and average axial temperature gradient of the ?30 mm cavity are 0. 093 and 2. 280 K/ mm, respectively. The simulation results reveal that the uniformity of the temperature field in the ?30 mm synthesis cavity is significantly better than that in the ?15 mm synthesis cavity. Secondly, the reproducible growth of high-quality type-Ib diamond large single crystals was achieved using both synthesis cavities mentioned above. The ?15 mm synthetic cavity is difficult to achieve the growth of high-quality type-Ib diamond single crystals weighing more than 1. 2 ct (1 ct = 0. 2 g), while the ?30 mm synthetic cavity is more suitable for the growth of large-size high-quality type-Ib diamond single crystals. Furthermore, the scanning electron microscopy ( SEM) test results show that the surface flatness of high-quality type-Ib diamond single crystals grown using two different sizes of synthesis cavities in this study is good, and the expansion of the synthesis cavity has no significant effect on the crystal quality of Ib diamond single crystal surface. Finally, Raman spectrum test results show that the crystal quality of polycrystalline diamond deteriorates, and other diamond single crystal samples synthesized by two kinds of synthesis cavity in this study have better crystal quality. This research has certain academic reference value for the design of large size synthesis cavity of gem grade diamond single crystal, the growth of large size diamond single crystal, and the perfection of polycrystalline diamond single crystal synthesis technology.