The invention discloses a method for preparing an AlN crystal, which relates to a method for preparing a crystal and solves the problems of large lattice mismatch between a heterogeneous seed crystal and the AlN crystal and high defect density of the AlN crystal in a method for preparing an AlN single crystal by a physical vapor transport (PVT) method in the prior art. The method comprises the following steps of: 1, putting AlN powder into a crucible, fixing a seed crystal on the top of the crucible, raising the temperature to 1,800-2,000 DEG C in the presence of nitrogen, and preserving the heat for 1 to 5 hours; and 2, heating to raise the temperature of the pre-sintered AlN powder to 2,150-2,300 DEG C in the presence of the nitrogen, preserving the heat, reacting for 8 to 20 hours, and reducing to room temperature. A zero micro tube SiC is used as the heterogeneous seed crystal, so that the defect density of the AlN crystal can be reduced; and simultaneously, an angle-deviated SiC seed crystal is deviated from a surface at a certain angel, so that the inheriting probability of defects is remarkably reduced, and negative influences of the defects on the performance of a device are reduced finally. The AlN crystal is used for a semiconductor device.