A composite wire includes a metal inner core, an easily-passivated metal layer wrapping a surface of the metal inner core, and a self-adhesive resin layer wrapping a surface of the easily-passivated metal layer. An insulation layer of the composite wire is a metal passivation layer that is formed by the easily-passivated metal layer obtained after sintering treatment and oxidation. The preparation method is used for manufacturing the composite wire. The method for preparing a power inductor is used for preparing a new type of power inductor including the composite wire. The composite wire is high-temperature resistant and is easily wound. During winding, the easily-passivated metal layer is unlikely to fall off, thereby ensuring that the insulation layer formed by passivation of the easily-passivated metal layer has desirable weather resistance and voltage resistance.