A method for preparing metallic magnesium surface ultra-hydrophobic water film comprises the following steps: pre-processing the magnesium base body to be disposed in the sulfuric acid solution to etch for 3-5 minutes, wherein the mass percent concentration is 1%; washing the base body with de-ionized water to be disposed in 10-20% of the hydrogen peroxide solution to oxidize for 1-3 minutes; washing the mixture with the de-ionized water to obtain the refined structure including the fresh oxidized film on the surface of the base body; immersing the magnesium base body including the surface refined structure in ethanol solution of 50mM stearic acid to self-assembly for 1-3 hours, taking out the mixture to dry at the room temperature to obtain the ultra-hydrophobic magnesium surface film. The preparing method has high-efficient technology, and easy-obtained, environment-friendly raw materials; and the method can be used for preventing pollution and rust of the metallic magnesium product in atmosphere environment.