Ce-Mn chemical conversion coating was prepared on AZ91 magnesium alloy at room temperature. The effects of Ce(NO3 )3 and KMnO4 concentrations on the corrosion resistance of conversion coating were investigated by the single factor experiment and the optimum concentrations were obtained. The conversion coating structures and compositions were studied. The anti-corrosion mechanism of Ce-Mn coating on magnesium alloy was investigated through electrochemical impedance spectroscopy (EIS). The results show that Ce-Mn conversion coating is mainly composed of oxide or hydroxide of Ce, Mn and little Mg with amorphous structure. The Ce-Mn conversion coating can play better anti-corrosion to magnesium alloy.