The invention relates to a preparation method of a diamond monocrystal cooling fin, and solves the problems that existing monocrystal diamond is high in chemical inertness and not easy to weld. The method comprises the steps of firstly using an ultrasonic wave cavitation erosion instrument, adopting a saline solution containing Fe3+ as a cavitation bubble production medium, and carrying out cavitation erosion treatment on the surface of a diamond monocrystal wafer; and then preparing a layer of strong carbide metal inner layer on the surface, preparing a layer of antioxidant outer layer, and finally carrying out solution treatment on the prepared diamond monocrystal wafer at high temperature. According to the method, a high-speed microjet produced during cavitation bubble collapse and burst at high speed is used for impacting the diamond monocrystal surface to cause local peeling so as to form tiny holes; a strong carbide metal atom can be reacted with a part of carbon atoms on the diamond surface so as to form chemical bond combination; and Au and a strong carbide metal element have favorable solid solubility and a favorable anti-oxidation property. Therefore, according to the prepared diamond monocrystal cooling fin, a coating and a substrate have high bonding strength, and the diamond monocrystal cooling fin has a favorable anti-oxidation property.