A metal/oxide three-layer heterojunction film and a preparation method thereof are disclosed, the method comprises the following steps: forming a platinum film, Yttrium iron garnet film and cobalt oxide film from bottom to top on a monocrystalline silicon or gadolinium gallium garnet substrate; Three-layer heterojunction structure was formed by coating with anti-ferromagnetic oxide CoO, and the traditional Pt film/YIG double-layer heterojunction structure was improved. Under the same thermal gradient condition, the spin thermoelectric voltage could be increased by 100%, which greatly increasesthe spin transmission rate and enhances the thermal stability of the device.