The invention provides a coating capable of improving the surface corrosion resistance and biocompatibility of medical magnesium and magnesium alloy and a preparing method of the coating. An electrolyte is composed of an aqueous solution containing Zn2+ salt, Ca2+ salt, H2PO4- salt and NaNO3. The zinc-calcium-phosphate-containing coating good in coverage is prepared on the surfaces of magnesium and magnesium alloy with the electrolyte through an electrochemical deposition method. The coating belongs to non-stoichiometric CaZn2(PO4)2.2H2O coatings, and has the significant advantages that degradation can be achieved while the corrosion resistance of a magnesium matrix is improved, and the degradability of the matrix cannot be changed by the coating. Meanwhile, Zn2+ can be released in the degrading process of the coating, differentiation of osteoblast can be promoted, the activity of osteoclast can be restrained, and thus the biocompatibility of the matrix is also improved. Thus, the coating has good application prospects when applied to magnesium alloy bone implanting medical instruments.