Abstract: The corrosion law and mechanism of AZ91D magnesium alloys were studied by means of optical microscopy, 3D digital microscopy, scanning electron microscopy, X-ray diffraction and self-made simulated acid rain spraying device. The results show that α phase in the alloy was severely corroded, β phase which contained high Al concentration was corroded slightly. The corrosion area could be divided into weak area and strong area. Pits were observed on the surface of the magnesium alloys at the initial period, which were caused by the dissolution of magnesium alloy matrix. As the corrosion going on, the surface was covered by the acicular products. Occluded area formed inside pits, which resulted in autocatalytic process.