The generation of sub-grains during the preparation of AlN crystals by physical vapor transport (PVT) method can reduce crystal quality, and even lead to the development of polycrystallization. In this paper, the generation and evolution of sub-grains were studied by adjusting the thermodynamic and kinetic growth conditions, and effective suppression methods were proposed to regulate crystal growth mode. The experimental results show that the large temperature fluctuation leads to the large fluctuation of the gaseous substances supersaturation on the crystal surface. It promotes the formation probability of high-index facets, which are difficult to achieve perfect coalescence due to non-uniformity. The influence of the transport path of gaseous substances on the crystal growth mode under the condition of a stable temperature field was also studied. The experimental and simulation results show that during the crystal growth under the traditional crucible structure, the transmission path of gaseous substances migrates from the edge to the center on the crystal surface, which is opposite to the direction of the growth step. The crystal surface is prone to forming step clusters and evolving into Stranski-Krastanov growth mode. A novel crucible was designed to enable the migration of gaseous substances along the crystal growth step direction, which met the basic conditions of layer-by-layer growth. The experimental results of growth temperature from 2 200 ℃ to 2 300 ℃ show that as the decrease of temperature, crystal growth exhibits c-axis dominant growth, and lateral expansion capability decreases. As the temperature increases, the lateral expansion capability increases, but the probability of the formation of high-index crystal facets increases continuously. When the temperature is about 2 250 ℃, the two reach a balance, which is suitable for the suppression and elimination of sub-grains. Combined with BCF theory analysis, reducing the supersaturation enhances the two-dimensional tiling ability of screw dislocation-driven growth, and increases the growth step width, which is conducive to dispersing sub-grain boundaries into step flow. The optimized experimental results show that the Stranski-Krastanov growth mode gradually changes to a screw dislocation-driven growth mode, and the sub-grains are gradually annihilated. The high-quality AlN crystals were obtained, with an X-ray single crystal rocking curve full width at half maximum values of 58″ on AlN (0002) surface, a Raman spectrum E2 (High) full width at half maximum value of 3.3 cm-1, and a dislocation density on the crystal surface of 2.87×103 cm-2.