The invention discloses a preparation of a high-quality AlN epitaxial layer. The preparation method comprises following steps of: (1) carrying out high-temperature baking on a substrate arranged in a reaction chamber; (2) pre-laying TMAl or TEA l before deposition of a nucleating layer; (3) depositing a low-temperature nucleating layer, wherein surface migration improving agent is injected into the low-temperature nucleating layer so that the size of crystal grains of the nucleating layer is adjusted; (4) carrying out temperature raising and annealing; and (5) growing the AlN epitaxial layer at a high temperature. According to the technical scheme provided by the invention, the preparation method is characterized in that the adjusting of the AlN nucleating layer is quite efficient and direct. By injecting a small quantity of the surface migration improving agent in the nucleating layer stage, the migration ability of diffusion atoms Al can be improved and the nucleation grain states of the AlN are changed. Compared with the prior art, the preparation method is characterized in that other additional effects caused by adjustment of temperature of the nucleating layer and flow of a reaction source can be avoided; the adjusting process is quite simple; and improving effects are remarkable.