Objective To evaluate the corrosion evolution behavior of black micro-arc oxide (MAO) film layer on magnesium alloy caused by chloride ion. Methods The electrochemical characteristics of the film were investigated using electrochemical polarization, electrochemical impedance spectrum (EIS) and scanning electron microscopy(SEM). Results The results showed that the potential and current were -1.464 V and 17.7 μA/cm2, while those of the micro-arc oxide specimens were reduced to -1.628 V and 0.09 μA/cm2, respectively. The addition of additive enhanced the corrosion properties compared to magnesium alloy. With increasing concentration of the additive, the corrosion property of the micro-arc oxide film first increased and then reduced. The optimized content was 10 g/L. The corrosion process of magnesium alloy could be divided into three processes. (1) Corrosive ions entered the film and caused changes of the melted interface. (2) The reaction happened between water and MgO, which led to density increase of the film. (3) Ions reached the magnesium and electrochemical corrosion occurred, forming wedge effect, which induced cracks and finally resulted in local corrosion failure. Conclusion The corrosion property of micro-arc oxide film was better than that of Mg alloy. The corrosion process could be divided to three stages: the entry of corrosive ions, reaction between water and MgO and local corrosion.