The effects of SO2 on the initial atmospheric corrosion of AZ91D magnesium alloy were investigated in laboratory. Metallographic observation, SEM (Scanning Electron Microscopy), XRD (X-ray Diffraction) and XPS (X-ray Proton Spectrograph) were used to analyze and discuss the initial surface morphology of corrosion layers and corrosion products. The corrosion rate of the alloy increases with increasing the content of SO2. The initial attack has the characteristics of localized corrosion and preferentially concentrates on a phase. MgO and Mg(OH)2 form at first, which provide a protective layer, then the existence of SO2 decreases the pH of the thin solution on the alloy, accelerates dissolution process, and promotes the formation of MgSO3.6H20 and MgSO4.6H20, meanwhile cracks were found on the corrosion products with corrosion continuation. These soluble corrosion products and the cracks provide the paths for filtering oxygen and corrosion pollutants into the matrix, which results in severe localized corrosion and the loss of protective function of film.