Elemental　Mg　is　easily　evaporated　or　burnt　during　welding　or　wire　+　arc　additive　manufacturing　(WAAM),　and　results　in　a　fluctuation　of　the　composition　and　mechanical　performances.　Elemental　Mg　loss　during　the　WAAM　of　Al-Mg　alloy　was　investigated　and　the　effect　of　Mg　loss　on　the　mechanical　properties　was　discussed　based　on　results　from　the　chemical　composition　measurement　and　mechanical　properties　test.　The　loss　of　elemental　Mg　was　non-negligible　because　of　remelting　during　WAAM,　and　the　loss　rate　of　elemental　Mg　increased　with　an　increase　in　current　and　decreased　with　an　increase　in　travel　speed.　The　elemental　Mg　distribution　in　the　WAAM　component　was　uniform,　but　obvious　element　enrichment　occurred　near　the　fusion　zone　of　the　substrate.　With　an　increase　in　the　loss　rate　of　elemental　Mg,　the　tensile　strength　and　average　hardness　of　the　WAAM　component　decreased,　whereas　the　elongation　increased.　During　the　WAAM　of　the　Al-Mg　alloy,　with　an　increase　in　the　Mg　loss　rate,　the　lattice　parameters　decreased　because　the　solid　solubility　decreased　in　the　Al　matrix　during　the　WAAM.