The invention discloses a method for preparing an ultra-flat copper monocrystalline film. The method for preparing the copper monocrystalline film, provided by the invention, comprises the following steps: taking a sapphire monocrystal as a growth substrate, carrying out copper target magnetron sputtering, and carrying out annealing, thereby obtaining the copper monocrystalline film. According to the method, sapphire is adopted as an epitaxial substrate of copper, a magnetron sputtering method is adopted, a copper film with consistent orientation is deposited on the surface of a c-face sapphire substrate, and copper with consistent orientation is aged and grows up during subsequent annealing so as to form a consistent-orientated monocrystalline copper (111) film without in-plane twin crystals. The monocrystalline copper (111) film prepared by the method is extremely flat in surface, controllable in diameter and high in repeatability and has a very extensive application prospect in the fields of communications, electronics, graphene preparation and the like.