The source voltage increment can influence the shape of arc spark and the thickness, surface morphology and corrosion rate of MAO coatings treated with silicate aqueous solution on AZ91D magnesium alloys. The results reveal that with the voltage increment of supply pulse increasing, the brightness and size of the arc sparks is increased but the amount is decreased accordingly, and the tendency of large arc discharge is intensified; the coating thickness is increased but the film growth rate is decreased; the surface melt grain, microspore size and surface roughness are increased; corrosion rate is first decreased and then increased. The comprehensive performances such as the process stability, film growth rate, coating appearance quality and corrosion resistance are relatively optimum when the voltage increment is within 100-150 V.