The invention relates to a method for preparing a layered double hydroxide (LDH) closing film layer on the surface of magnesium alloy. According to the method, anodized magnesium is directly used as a magnesium source, and the LDH closing film layer is grown in situ on the surface of the anodized magnesium alloy through in-situ growth fluid. Holes in the surface of a magnesium alloy anodized film can be effectively closed, direct contact between corrosive media and a magnesium alloy substrate is avoided, the thickness of a protection layer is increased, and the corrosion resistance of the protection layer is obviously improved. An inorganic salt type reagent is adopted as the reagent of the method, environmental friendliness is achieved, and the reagent is sold in the market, so that the cost is low.