The invention discloses a method of preparing a fin-type three-dimensional multiferroic heterojunction by adopting an atomic layer deposition method. A micro nano processing technology is used for processing a ferroelectric single crystal substrate to a fin-type three-dimensional nano structure, excellent three-dimensional shape-preserving uniformity of atomic layer deposition is used, ferrocene and oxygen with low price serve as iron and oxygen precursor sources, an atomic layer deposition ALD device is used for in-situ growing a 100% uniform and shape-preserving Fe3O4 thin film on the ferroelectric single crystal with the fin-type three-dimensional structure, and the fin-type three-dimensional multiferroic heterojunction is formed. the micro nano processing technology and the atomic layer deposition technology are organically combined, the limitation that the existing multiferroic heterojunction still stays in a plane structure and a macro dimension are broken through, and an important role is played in three-dimension and miniaturization of the multiferroic heterojunction and compatibility with the existing mainstream microelectronic device.