To improve the corrosion resistance of Mg-RE alloy, micro-arc oxidation coating was formed on the surface of the Mg-RE alloy. The Mg-Gd-Y alloy silicate microarc oxide coating was subsequently post-treated with a phosphate post-treatment solution to seal the holes, and the corrosion inhibitor was added in the process. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to analyze the surface morphology and composition of the coating. The polarization curve and electrochemical impedance spectroscopy (EIS) were used to test the corrosion resistance of the coating. Post-treatment can form a MgHPO4 deposition layer on the surface of the micro-arc oxidation coating. The generation of the deposition layer effectively sealed the micro-holes, cracks and other defects on the surface of the micro-arc oxidation coating. The addition of corrosion inhibitors significantly increased the amount of deposits, which increased the phosphorus content of the coating from 5.37% to 14.90%, which significantly enhanced the deposition. The polarization experiment shows that the corrosion current density of the coating after post-treatment decreased from 1.51×10–7 A/cm2 to 4.91×10–8 A/cm2. The corrosion current density is further reduced to 5.76×10–9 A/cm2 after loading the corrosion inhibitor, indicating that the corrosion resistance is significantly improved. After the coating is soaked in 3.5 wt.% NaCl solution for 384 hours, the total impedance of the post-treatment coating loaded with corrosion inhibitor can reach to 7825.3 Ω.cm2, which is significantly higher than the 403 Ω.cm2 of the micro-arc oxidation coating. It can effectively improve the corrosion resistance of the micro-arc oxidation coating. Furthermore, when the high efficiency corrosion inhibitor SDS was added, the resistance increased tenfold. The resistance of the sealed MAO coating with inhibitor increased from 0~48 h, which is related to the release of the inhibitor. The gradual release of the inhibitor guarantees a long-time corrosion resistance of the coating.