The invention discloses a method for preparing an iron phthalocyanine monocrystal thin film on a graphene substrate and belongs to the technical field of organic semiconductors. The method comprises:preparing a single layer of monocrystal graphene on a monocrystal nickel sheet having a crystal plane (111) by a chemical vapor deposition method to obtain a nickel-based graphene substrate, preparingan iron phthalocyanine monocrystal thin film on the nickel-based graphene substrate through a thermal deposition method, and depositing the iron phthalocyanine monocrystal thin film on the nickel-based graphene substrate. Through the method, iron phthalocyanine molecules grow along the specific direction so that an organic nano-crystal domain having nano-linear morphology is formed and the iron phthalocyanine molecules are distributed on the nickel-based graphene substrate in a lying form so that a structurally ordered monocrystal film is formed. Through the electron coupling between metallicnickel and graphene and interaction between the electron coupling system and the iron atom non-closed shell d electrons, the organic nano-crystal domain having nano-linear morphology is prepared at an appropriate deposition rate.