The surface appearance and corrosion products of Zn-Mg alloy coatings with different content of Mg were analyzed by SEM and XRD.As a result,crystal particles of coatings were being diminished and structure tends to uniformity with growing Mg content.However,when the content of Mg was 2.5%,there will be Mg segregated in crystal interface.When the content of Mg was 3.0%,there will be more Mg segregated in crystal interface.That was approved by electronic probe.The baseline of diffraction was lifted and diffraction peak was broaden with growing Mg content.That was the content of non-crystal material in corrosion products grew,however,there was MgO in coating corrosion products when the content of Mg was 3.0%.It can be analyzed from the corrosion theory that the cathodic protection corrosion products were formatted by the segregation of Mg,the oxide film containing MgO was loose and cavernous and its compacting factor was 0.79.The reason that the ability to resist corrosion decreased was the chemical composition and structure of the film could not inhibit corrosion medium and corrosion.