AZ91D magnesium alloys were immersed in different phosphating solutions with zinc nitrate and sodium fluoride additives to enhance the corrosion resistance. The devolution law of the Open Circuit Potential (OCP) of AZ91D alloys during phosphating was measured. The corrosion behaviors of AZ91D alloys in 3.5%NaCl solution were investigated using OCP and Tafel polarization methods, and the effect of phosphating additives was discussed. The results show that the changes of the OCP of AZ91D alloys with phosphating time in different phosphating solutions are different; the anodic and cathodic corrosion processes of AZ91D alloys are conspicuously inhibited with phosphate coatings; zinc nitrate and sodium fluoride are benefit to form phosphate coatings with better corrosion resistance. The corrosion potential of AZ91D alloy phosphated in solutions with both zinc nitrate and sodium fluoride is most positive.