The invention relates to a controlled degradation magnesium alloy coating stent and a preparation method. The stent body is made of medical high purity magnesium or magnesium alloy by mechanical processing or laser carving; the stent body is provided with a drug-loading coating which bears curative drug; the surface of the stent body is provided with an anti-corrosive coating; the surface of the anti-corrosive coating is provided with a degradable polymer film drug-loading coating; the preparation method includes surface cleaning, preparation of the degradable polymer film drug-loading coating, and application of curative drug; through (1)surface cleaning, (2)preparation of the degradable polymer film drug-loading coating, and (3)application of curative drug, an oxide film is formed on the surface; different drugs and dosage can be fixed by regulating the molecular weight and the thickness of the polymer layer, the drug-loading quantity is more than 30 percent, which improves the fixed stability of the drug, greatly reduces the degradation speed of the magnesium alloy and controls release of the drug, delays corrosion of the magnesium alloy, extends the service life of the stent, is safe in use, and meets the clinical requirement.