Magnesium alloys were treated in a solution containing silicate, fluoride and glycerol at a constant applied current density. The effect of current density on breakdown voltage, ignition time and coating thickness was investigated. SEM was employed to examine the surface morphology of ceramic coatings formed on magnesium alloys at different applied current density, and the corrosion resistance was also measured by EIS. It was concluded that according to the Voltage-Time response, the MAO process is simply divided into gradually increasing stage and a relative stable stage. With the increase of applied current density the cell voltage increasing rate is improved, coating thickness is increased lineally and ignition time is decreased, but the breakdown voltage has no apparent variation. The number of pores is decreased with the increase of current density, but the diameters of the pores are increased; EIS results indicate that corrosion resistance of ceramic coating increase firstly, then decrease with the increase of applied current density.