The invention discloses a preparation method of a metal-matrix nanocomposite, and belongs to the field of metal material preparation. According to the preparation method of the metal-matrix nanocomposite, high-content and evenly distributed nano-reinforcement bodies are added into target metal through molten salt assisting, thus the hot-crack tendency of matrix metal is reduced and the forming property of the matrix metal is improved while the matrix structure is refined and matrix strength is improved. The prepared metal-matrix nanocomposite can be widely applied to the field of casting, welding and 3D printing.