In the magnesium air battery, the polarization and corrosion of the magnesium anode is an important index to measure the magnesium air battery. In this paper, by means of dynamic potential polarization method and electrochemical impedance spectroscopy (NaCl), the effect of six hydrated cerium (CeN₃O₉.6H₂O) on the corrosion resistance of AZ31 cathode was studied. The results showed that CeN₃O₉.6H₂O were added into the electrolyte to form Ce(OH)₃ protective film on the surface of AZ31 magnesium alloy to improve the corrosion resistance of the magnesium alloy. With the increase of the concentration of CeN₃O₉.6H₂O, the Ce(OH)₃ protective film became denser, and the corrosion rate of AZ magnesium alloy was decreased. When the concentration of CeN₃O₉.6H₂O reached 1.0g/L, the corrosion rate of the magnesium alloy was the lowest, and the corrosion inhibition rate was 70.4%. However, when the concentration of CeN₃O₉.6H₂O is greater than 1.0g/L, the corrosion rate of magnesium alloy is increased due to the dissolution of Ce(OH)₃ protective film. Ce(HO)₃ protective film formed on the surface of magnesium alloy CeN₃O₉.6H₂O was found by the SEM diagram of AZ alloy, which was found in the of 50h after immersion. The Mg₂₊ charge transfer resistance is increased by 69.5Ω from the equivalent circuit diagram, which improves the corrosion resistance of the magnesium alloy. The discharge performance of CeN₃O₉.6H₂O was improved by the discharge test, and the discharge time was prolonged by nearly 1h.