With scanning Kelvin probe and microscopy technique, the degradation process of the micro-arc oxidation (MAO) treated magnesium alloy was investigated under a 3% NaCl droplet. Because of the unique two-layer structure of the MAO specimen, the droplet penetrated in and spread through the external loose layer during the initial several minutes. Subsequently, with the increase of time, only a thin layer of electrolyte was left on the surface of the MAO specimen. During the entire corrosion process, the potential exhibited a valley type distribution, only the width of potential valley increased with time. Based on above observations, a model was proposed for describing the contamination mechanism of MAO treated magnesium alloy under droplet.