In　order　to　monitor　the　early　fatigue　damage　of　AZ31　magnesium　alloy　by　nonlinear　ultrasonic,　a　nonlinear　FEM　model　is　established　by　a　special　element　which　accounts　for　a　nonlinear　stress-strain　relation.　The　influence　of　the　micro-defect　with　different　length,　quantity　and　width　to　ultrasonic　nonlinearity　parameters　is　calculated.　The　simulation　results　show　that　micro-defects　in　material　fatigue　is　the　cause　of　the　second　harmonic　generation,　finite　element　method　can　effectively　simulate　ultrasonic　nonlinear　effects　in　magnesium　alloy　material.　Meanwhile,　ultrasonic　nonlinearity　parameters　of　two　magnesium　samples　are　online　measured.　There　is　a　significant　increase　inlinked　to　fatigue　cycles　in　early　stages　of　fatigue　life,　but　at　the　late　stage　of　fatigue,　an　experimental　phenomenon　of　the　decrease　in　fatigue　cycles　is　obtained.　The　experimental　results　show　that　ultrasonic　nonlinearity　parameters　can　characterize　the　early　fatigue　damage　of　magnesium.　The　results　of　simulation　verify　experimental　results.