Provided are a coating capable of improving the surface corrosion resistance and biocompatibility of medical magnesium and magnesium alloys and a preparation method therefor. In the present invention, an electrolyte is composed of an aqueous solution containing a Zn2+ salt, a Ca2+ salt, a H2PO4 - salt and NaNO3. A zinc-calcium-phosphate-containing coating with good coverage is prepared on the surfaces of magnesium and a magnesium alloy with the electrolyte through an electrochemical deposition method. The coating of the present invention falls within non-stoichiometric CaZn2(PO4)2·2H2O coatings, and is mainly applied to medical magnesium and magnesium alloys, especially for the surface treatment of a magnesium alloy for bone implant materials. The significant advantages thereof are that degradation can be achieved while the corrosion resistance of a magnesium matrix is improved, and the presence of the coating does not change the degradability of the matrix. At the same time, Zn2+ can be released during the degradation of the coating, the differentiation of osteoblasts can be promoted, and the activity of osteoclasts can be inhibited, such that the biocompatibility of the matrix is also improved. Thus, the coating has good application prospect in magnesium alloy bone implant medical instruments.