Wire　arc　additive　manufacturing　technology　has　many　advantages　in　producing　large　engineering　structures　and　is　being　applied　in　aerospace　structures.　It　is　necessary　to　explore　the　fatigue　failure　mechanism　of　wire　arc　additive　manufactured　aerospace　aluminum　components　to　ensure　these　structures＇　safe　application.　Crack　initiation　and　propagation　behavior　for　wire　arc　additive　manufactured　Al-Mg　alloy　are　investigated　under　fatigue　loading.　The　microstructure　characteristics　of　different　regions　are　analyzed　by　metallographic　and　electron　back-scattered　diffraction　measurements.　The　fatigue　test　results　show　that　multi-crack　initiation　is　the　fatigue　damage　characteristic　for　the　additive　manufactured　parts.　The　small　cracks　are　distributed　in　the　deposition　and　layer　band　zones,　and　the　number　of　small　cracks　increases　with　the　cyclic　numbers.　The　main　crack　initiates　from　the　coarse　columnar　grain　boundaries　in　the　deposition　zone,　and　the　final　fracture　is　in　the　strain　concentration　zone.　The　fatigue　fracture　morphologies　show　that　the　fatigue　cracks　initiate　at　the　pores,　non-melted　particles,　and　grain　boundaries.