Abstract: The disclosure describes a topological insulator having a new surface electronic state and a preparation method thereof, and more particularly, to a topological insulator having a new surface electronic state, the topological insulator including a unimolecular metal layer formed on a 3D topological insulator, and a method of preparing a topological insulator having a new surface electronic state, the method including: heating and cooling at least one selected from the group consisting of tellurium (Te) and selenium (Se), and bismuth (Bi) to prepare an alloy; and forming a unimolecular metal layer on the alloy.