Tafel polarization curves and electrochemical impedance spectroscopy (EIS) techniques were used to study the inhibition of corrosion of AZ40 magnesium alloy by molybdate in simulated cooling water.The surface morphology and elemental composition of AZ40 magnesium alloy exposed in the simulated cooling water with or without molybdate were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).The results showed that the corrosion of magnesium alloy in simulated cooling water was inhibited by addition of molybdate.The highest inhibition efficiency was achieved when the concentration of molybdate was 1000 μg·g-1.Molybdate mainly inhibited the anodic reactions of magnesium alloy through weakening the adsorption of aggressive Cl- on the alloy/medium interface and forming a compact protective layer that covered the magnesium alloy surface active sites.