The corrosion behavior of AZ91D magnesium alloy was studied in atmosphere containing 4×10-6 SO2 (volume fraction) by indoor simulation experiment. Corrosion morphology was observed by atomic force microscopy (AFM), optical microscopy (OM) and scanning electron microscopy (SEM). Corrosion products were analyzed by X-ray diffraction (XRD). The mass gain of AZ91D magnesium alloy during the corrosion period of 1200 h can be divided into two stages. The first stage accords with linear increase law, and the second stage accords with exponential decrease law. β phase is protected as cathode, and granular corrosion products are formed in α phase as anode. The corrosion products grow outward until the continuous corrosion layer is formed. Meanwhile, large granular corrosion products appear in the cracks of corrosion layer. The corrosion products mainly consist of MgO, Mg(OH)2, MgSO3·6H2O and MgSO4·6H2O. The quantity of MgO and Mg(OH)2 is decreased, while the quantity of MgSO3·6H2O and MgSO4·6H2O is increased for a longer corrosion time